Flatten Nested List Iterator
The problem involves implementing an iterator to flatten a nested list of integers. Below, I’ll provide a straightforward implementation of the NestedIterator
class.
Initialization: In the constructor, we need to convert the nested list into a flat list that we can iterate over. We’ll use a recursive helper function to do this.
Next: Simply return the next integer from the flattened list.
HasNext: Check if there are any more integers in the flattened list.


Explanation:
 In the
__init__
method, we’re recursively flattening the nested list intoself.flat_list
.  The
next
method returns the next integer fromself.flat_list
.  The
hasNext
method checks if there are more integers to be iterated.
This code ensures that the iterator behaves according to the specified requirements, returning the elements of the nested list in the correct order.


10 Prerequisite LeetCode Problems
This requires understanding of Nested Integer Lists, Iterators, and possibly the use of Stack/Queue. Here are 10 problems to prepare for this kind of problem:
102. Binary Tree Level Order Traversal: This problem can help you get familiar with the idea of dealing with nested structures in a stepbystep manner.
103. Binary Tree Zigzag Level Order Traversal: This problem requires a similar level of understanding of nested structures, but introduces an additional complexity with the zigzag pattern.
94. Binary Tree Inorder Traversal: This problem will help you understand how to use a stack to traverse a tree, which could be useful if you decide to use a stack to solve the Nested List Iterator problem.
144. Binary Tree Preorder Traversal: This problem, like the Inorder Traversal problem, involves stack usage and can provide good practice for tree traversal.
155. Min Stack: This problem can help you practice implementing a stack with additional features.
225. Implement Stack using Queues: This problem can provide practice with basic stack operations and understanding the differences between stacks and queues.
232. Implement Queue using Stacks: This problem is a companion to the previous problem, and involves implementing a queue using stacks.
282. Expression Add Operators: This problem is more complex, but can help you practice dealing with nested structures and building them in a controlled manner.
385. Mini Parser: This problem, like the problem you’re working on, involves dealing with NestedInteger objects, and can provide helpful practice.
341. Flatten Nested List Iterator: Trying out the problem itself, understanding the constraints, and attempting to solve it will often provide the most effective practice.
Problem Analysis and Key Insights
Here are some key insights gained from analyzing this nested iterator problem:
The nested structure represents a tree, where child lists are attached to parent integer nodes.
We need to traverse this tree depthfirst to generate the flattened sequence.
An iterator interface needs to be implemented with hasNext() and next() methods.
hasNext() should return true if any nodes are left in the traversal.
next() should return the integers in the order visited by the traversal.
The traversal order and output sequence matter, not just visiting all nodes.
No specifics given on efficiency requirements, so multiple traversal approaches may be viable.
The problem seems selfcontained  no external data or states beyond the nested list input.
In summary, modeling the nesting as a tree and leveraging tree traversal algorithms while implementing the iterator interface seem like good fits based on the insights from the problem statement.
Problem Boundary
Here are some key points about the scope of the nested iterator problem:
The input is a nested list structure containing integers and lists.
The total number of elements is reasonably bounded between 1500.
Integers are within a reasonable range of 106 to 106.
The nesting structure forms a tree, with lists as children attached to integer nodes.
An iterator must be implemented with hasNext() and next() methods.
Output is a flattened sequence of all integer nodes. Order matters.
No other operations or analyses on the structure are required.
Efficiency requirements are unspecified, so multiple traversal approaches could be valid.
The nested list input is provided  no need to handle input creation or validation.
So in summary, the scope is focused on implementing iterator methods to traverse the nested structure and output the integers in correct sequential order. Efficiency and actual construction of the nested list input are not specified.
Here are some ways we can establish boundaries for this nested iterator problem:
Input Space
 Nested list of integers and lists
 Up to 500 total elements
 Integers from 106 to 106
 Nesting depth unspecified
 Structure forms a tree
Output Space
 Linear sequence of integers
 Ordered by traversal
Functionality
 Implement hasNext() and next() methods
 hasNext() returns Boolean
 next() returns integers
 Correct traversal order
Efficiency
 No specific time or space complexity bounds given
Errors
 Invalid nesting
 Integer outside bounds
 Calling next() with no elements left
Out of Scope
 Actual construction of nested structure
 Deleting/inserting nodes
 Tracking additional metadata
Defining these clear input, output, functionality, and error handling expectations provides a solid problem specification to design the iterator within.
Problem Classification
This is a data structures and algorithms problem in the domain of computer science.
The ‘What’ components are:
 Nested list of integers as input
 Integers can be nested inside lists within the nested list
 Implements an iterator interface for the nested list
 Iterator has next() and hasNext() methods
 next() returns each integer in flattened sequence
 hasNext() indicates if integers left
Based on this, I would categorize this problem as:
 Iterator design problem, since we need to implement iterator interface
 Tree/graph problem, as the nested structure forms a tree
 Tree traversal problem, as we need to traverse the nested structure
 Flatten/linearize problem, since we convert the nested structure to a flat sequence
The core is implementing iterator logic that can traverse the nesting structure and linearize it. This requires modeling the nested list as a tree and leveraging tree traversal algorithms.
I would classify this as an iterator design and tree traversal problem to flatten a nested structure.
Distilling the Problem to Its Core Elements
This nested iterator problem is fundamentally based on the concepts of tree data structures and traversal algorithms.
In the simplest terms, I would describe it as:
“Traversing a nested list structure in the right order to flatten it.”
The core problem is flattening a nesting structure by visiting all its nodes in an appropriate sequence. We can simplify it to:
“Flatten nested list using traversal ordering.”
The key components are:
 Nested list as tree structure
 Depthfirst traversal
 Tracking visited nodes
 Iterator interface
The minimum operations needed are:
 Model nested list as tree
 Implement DFS traversal
 hasNext() checks unvisited nodes left
 next() returns current node
At its essence, this requires representing the nesting as a tree and leveraging tree traversal algorithms to visit nodes in the right order while implementing the iterator interface. The core is flattening via structured traversal.
Visual Model of the Problem
Here are some ways to visualize the nested iterator problem statement:
Draw the nested list as a simple tree with integer nodes and lists as branches.
Animate a depthfirst traversal that visits each node, numbering them in order.
Highlight the node sequence produced by the traversal.
Show this final sequence as the flattened result.
Visualize calls to hasNext() checking if nodes left unvisited.
Show next() advancing traversal and returning current integer node.
For larger trees, draw top levels fully but summarize subtrees as branches.
Use colors to distinguish between integer nodes and list nodes.
Illustrate invalid traversals that produce incorrect order.
These types of tree drawings, animations, and annotations help provide an intuitive sense of the core concepts like nested structure, traversal ordering, node sequencing, and implementing iterator methods. Visuals make the problem more concrete.
Problem Restatement
Here’s how I would paraphrase the nested iterator problem statement in my own words:
We are given a nested list structure containing integers and other nested lists at arbitrary levels. This forms a treelike structure where lists branch off from integers.
We need to implement an iterator for this structure with two methods:
hasNext() should return true if there are still unvisited integers left in the structure based on a depthfirst traversal order.
next() should return the integers of the structure in the order that they would be visited in a depthfirst traversal.
The goal is to “flatten” out the nested structure by iterating through it correctly using these two methods. The order in which integers are returned matters.
The total number of elements in the nested list is reasonably bounded between 1500. The integer values are also in a reasonable range.
No other specifics are provided on efficiency requirements or other functionality needed.
Abstract Representation of the Problem
Here is one way to formulate an abstract representation of this nested iterator problem:
Let’s consider an abstract tree structure T consisting of:
 Node objects N
 Edges E connecting the nodes
Where T has the following properties:
 There are two types of nodes: integer nodes and list nodes
 List nodes have edges pointing to child nodes
 Integer nodes contain integer values
We want to define an Iterator I for T that:
 Has a next() method that returns the next integer node by traversing T
 Has a hasNext() boolean method indicating if integers left
The traversal order of I should be:
 Depthfirst traversal of T
By abstracting away the specifics of nested lists and integers, we can frame this problem as:
 Defining an iterator I for a tree T
 I provides sequential access to T’s nodes
 I traverses T using a depthfirst ordering
This focuses the problem on iterating through a tree structure correctly based on the ordering needs, without domainspecific details.
Terminology
Here are some key terms and concepts important for understanding this nested iterator problem:
Nested structure  Structure composed of elements containing subelements nested within them. Forms a tree.
Tree  Hierarchical abstract data structure with a root node and subtrees of children. Models nesting.
Depthfirst search (DFS)  Tree traversal algorithm that visits nodes and fully explores subtrees before siblings. Produces correct output order here.
Iterator  Object providing sequential access to elements of a container via next() and hasNext() methods. Specified interface.
Traversal order  Order in which nodes are visited in a tree traversal. Affects output sequence.
Flattening  Converting a nested structure to a flat linear sequence. The problem’s goal.
Time complexity  Runtime as a function of input size. Not specified but can analyze.
Space complexity  Memory usage as a function of input. Can also analyze if needed.
Understanding tree data structures and traversal algorithms is key to modeling the nesting and generating the flattened output sequence. Implementing the iterator interface enables exposing this traversal sequentially.
Problem Simplification and Explanation
Here’s one way I could explain this nested iterator problem in simpler terms:
Let’s compare the nested structure to a corporate organization chart (analogy). There are managers (integer nodes) and under them are subteams (list nodes).
The key concepts are:
 Managers = integer nodes
 Subteams = list nodes
 Hierarchical structure = nested tree
We want to traverse the org chart visiting all managers in the right order such that:
 We know if any unvisited managers remain (hasNext())
 We can access the next manager in the traversal sequence (next())
Proper order here is depthfirst, fully exploring subtrees before moving to siblings.
So essentially we need to flatten the org chart into a sequenced list of managers using a depthfirst traversal ordering, while implementing iterator methods to expose that sequencing.
Let me know if this analogy helps relate the key concepts or if any part needs more clarification!
Constraints
Here are some characteristics of this nested iterator problem that we can potentially leverage:
The small maximum size of 500 elements allows simpler traversal algorithms without needing complex optimizations.
Integer values are within a reasonable small range. This avoids overhead of variablelength encodings.
Nesting depth is unspecified, but small total size limits maximum depth.
No filtering or processing of nodes is required  we visit every node.
Output is linear sequence of integers. We don’t need to reconstruct structure.
No efficiency requirements are specified, so we can focus on cleanest traversal algorithm.
The structure is static  we don’t need to account for online mutations.
No constraints on memory usage, allowing traversal algorithms requiring stacks.
Overall, the small discrete problem size, reasonable value bounds, lack of efficiency requirements and simple output allow us flexibility to select a clean recursive depthfirst traversal approach without needing to optimize performance or storage.
Here are some key insights gained from analyzing the constraints:
The small scale of the input allows simple traversal algorithms without complex optimizations.
Bounded integer values simplify storage and access without concern for variable encodings.
Unspecified nesting depth limits maximum depth given bounded total size.
Visiting every node simplifies logic compared to filtered traversals.
Linear integer output removes any subtree reconstruction overhead.
No efficiency requirements provides leeway to focus on cleanest algorithm.
Static structure means we don’t have to account for ongoing changes during traversal.
Unlimited memory permits use of naive recursion instead of explicit stacks.
In summary, the constraints enable a simple, recursive depthfirst traversal approach without needing to optimize performance, storage, handle mutations, or reconstruct state. This provides flexibility to use a clean algorithm.
Case Analysis
Here are some additional test cases covering different scenarios:
Basic case
Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Reasoning: Tests small sample structure and ordering.
Single integer
Input: [1]
Output: [1]
Reasoning: Edge case with just one integer.
All nested lists
Input: [[[],[]], [[]]]
Output: []
Reasoning: No integers case.
Large depth
Input: Nested list with depth 10
Output: Correct sequential traversal
Reasoning: Stress tests recursion depth.
Large breadth
Input: 500 element shallow nested list
Output: Proper traversal order
Reasoning: Stress tests iterator logic.
Edge cases:
 Empty nested list
 One integer
 No integers
 Max depth
 Max total elements
Testing these provides coverage over different shapes, sizes, depths, and integer distributions  helping validate correctness.
Here are some key insights gained from analyzing these nested iterator test cases:
The basic case validates the traversal order and iterator logic works on a simple example.
The single integer case reveals assumptions about requiring nested lists.
The no integers case handles another edge assumption on input structure.
Large depth cases stress test recursion stack and handling.
Large breadth cases validate performance for many elements.
Empty and single element cases verify handling of minimial structures.
Max depth and size cases check performance bounds.
A diversity of structures with different integer distributions is needed.
Both depth and breadth should be stressed independently.
Overall, these insights show that a wide variety of test cases are crucial for thoroughly exercising all aspects of the traversal logic and iterator handling over the full spectrum of valid input shapes, sizes and configurations.
Identification of Applicable Theoretical Concepts
Here are some mathematical and algorithmic concepts that could help optimize this nested iterator problem:
Tree data structures  Modeling the nesting as a tree enables using traversal algorithms.
Depthfirst search  Produces the correct traversal ordering for flattening.
Stack/Queue  Can explicitly handle recursion stack with a stack/queue.
Recursion  More elegant depthfirst traversal than explicit stack.
Iterators  Separating traversal from access logic via iterator pattern.
Lazy evaluation  Avoid traversing entire tree upfront. Compute nodes on access.
Memoization  Cache already visited nodes in hash table.
Graph theory  View as graph problem by modeling nodes/edges.
Preorder/Postorder  Variations produce different traversal orderings.
Modeling as a tree and leveraging concepts from algorithms, data structures, and design patterns provides a concise, efficient solution.
Simple Explanation
Here’s how I would explain this nested iterator problem in simple terms:
Let’s imagine you have a big family tree showing all the connections between parents, grandparents, kids, cousins, etc (metaphor).
Some family members have lots of children, while others have none. So the tree branches out unevenly in places.
We want to be able to flatten out this family tree into a simple list of all the family members that we can iterate through.
But we want the family members to be listed in a specific order  parents should come before their children, and everyone in one generation should come before the next generation.
So we need to traverse the tree in an ordered way so that when we iterate through the flattened list, family members appear in the proper generational sequence.
We also want a way to check if there are still more family members left undiscovered in the tree.
This is like flattening out a nested structure while retaining the original relationships in the order you traverse it. We use the iterator to sequentially access the flattened sequence.
Problem Breakdown and Solution Methodology
Here is a detailed stepbystep approach I would take to solve this nested iterator problem:
Model the nested structure as a tree, with integer nodes and list nodes containing nested child subtrees.
Implement a depthfirst traversal recursively, tracking visited nodes. If an unvisited node is found, increment depth and recurse on its children.
Maintain state of the inprogress traversal, including current node, nodes left to visit, and next integer to return.
hasNext() checks if there are any unvisited nodes remaining by consulting the traversal state.
next() uses the traversal state to return the next integer node from the depthfirst sequence.
Incrementally traverse the tree only when hasNext() or next() are called to lazily generate the sequence.
Increasing nesting depth could require tuning recursion stack size and heap usage. We can optimize by pruning redundant traversals.
Let’s walk through the simple nested list [[1,1],2,[1,1]]:
 Depthfirst order: 1, 1, 2, 1, 1
 hasNext() returns true until final 1 visited
 next() sequentially returns integers 1, 1, 2, 1, 1
This leverages depthfirst traversal to flatten the structure while implementing the iterator interface for sequential access.
Inference of ProblemSolving Approach from the Problem Statement
Here are the key terms and how they guide the solution approach:
Nested structure  Suggests modeling as a tree.
Iterator interface  Need to implement hasNext() and next() methods that provide sequential access.
Depthfirst traversal  Produces ordering needed to return integers through iterator.
Flattening  Goal is to flatten nested structure into linear sequence.
Tree/Graph  Nested structure can be represented as nodes and edges. Enables traversal algorithms.
Recursion  Concise way to implement depthfirst traversal on tree.
Lazy evaluation  Avoid eager traversal, compute nodes on access for efficiency.
The nested structure, iterator interface specification, and flattening goal motivate modeling the input as a tree and leveraging depthfirst traversal to lazily produce the sequential output. This guides the usage of tree algorithms and recursion to traverse the structure only as needed.
Here are some ways to visualize the properties and concepts of this problem using diagrams:
Nested Structure as Tree
 Draw nested lists as child nodes attached to integer nodes
 Use tree structure to represent nesting relationships
DepthFirst Traversal Order
 Animate traversal visiting nodes in DFS order
 Number nodes in order visited
Iterator Methods
 Show hasNext() checking state for unvisited nodes
 Illustrate next() advancing traversal state
Lazy Evaluation
 Traverse subtree only when expanded in visualization
 Leave subtrees summarized pretraversal
Recursion Stack
 Show recursive calls as stacking frames
 Pop frames off as recursion unwinds
These types of visual models reinforce concepts like representing nesting as a tree, traversing lazily depthfirst via recursion, and implementing external iterator methods. Diagrams provide an intuitive aid.
How did you infer from the problem statement that this problem can be solved using ?
Simple Explanation of the Proof
I’m having trouble understanding the proof of this algorithm. Could you explain it in a way that’s easy to understand?
Stepwise Refinement
Here is a stepwise refinement for this nested iterator problem:
Highlevel approach: Use depthfirst traversal to flatten nested structure.
Break into steps:
 Model nested list as tree
 Implement depthfirst traversal logic
 Maintain state of iteration
 hasNext() checks unvisited nodes left
 next() returns next integer in sequence
 Only traverse when accessed
 Independent subproblems:
 Building tree structure
 Tracking visited state
 Traversal sequence generation
 Repeatable patterns:
 Recursive DFS calls on unvisited children
 Updating traversal state on node visits
 Checking state to handle iterator methods
The key is representing the nesting as a tree and using depthfirst search to traverse it, implementing the iterator on top of the traversal state. This breaks down the problem into modular components.
Solution Approach and Analysis
Here is a detailed stepbystep approach to solving the nested iterator problem:
Model the nested list as a tree with integer nodes and list nodes containing child subtrees.
Maintain state of current position in tree and nodes visited.
Implement a recursive depthfirst traversal that lazily visits unexplored subtrees when accessed.
hasNext() checks if any nodes are unvisited by consulting the traversal state.
next() uses state to find and return next integer node in depthfirst sequence.
Update traversal state as nodes are visited.
Only traverse subtrees when their parent node is accessed, for lazy evaluation.
Increasing nesting depth could require tuning stack size. We can optimize by pruning redundant traversals.
Let’s walk through nested list [1, [2], 3]:
 hasNext() is true
 next() returns 1
 hasNext() is true
 next() returns 2
 hasNext() is true
 next() returns 3
 hasNext() is false
This implements lazy depthfirst traversal to flatten on demand while exposing it through the iterator interface.
Identify Invariant
The invariant in this nested iterator problem is:
At any point during traversal, the iterator’s state accurately reflects the current position within the depthfirst traversal sequence of the tree.
This means the state which tracks visited nodes, current node, and unvisited child subtrees must properly capture:
 What nodes have been visited
 Where the traversal currently is
 What nodes are left to traverse
According to the ordering of a depthfirst traversal.
We can maintain this by:
 Initializing the state when starting
 Updating it appropriately on node visits
 Consulting it when implementing hasNext() and next()
This ensures hasNext() and next() behave correctly throughout the iteration sequence.
The invariant connects the private traversal state to the public iterator interface exposed externally.
Identify Loop Invariant
What is the loop invariant in this problem?
Thought Process
Here is how I would approach solving this nested iterator problem:
The cues from the problem statement:
 Nested list structure provided
 Need to implement iterator interface
 Iterator should flatten structure
 Must return integers in correct order
This suggests:
 Model the nesting as a tree
 Use depthfirst traversal to visit nodes
 Maintain state of traversal order
 Implement hasNext() and next() via state
Steps:
 Represent nested structure as tree
 Track visited nodes
 Recursive depthfirst traversal
 hasNext() checks unvisited nodes remaining
 next() gets next integer from traversal state
 Only traverse when accessed
Python code:


This models the nesting as a tree, implements depthfirst traversal, and leverages the state to provide the iterator interface.
Establishing Preconditions and Postconditions
Here are the details for the hasNext() method:
Parameters: None
Preconditions:
 Nested iterator is initialized
 Tree is built
 Functionality:
 Checks if any unvisited nodes left in traversal
 Postconditions:
 Returns True if nodes left, False otherwise
 Error handling:
 If iterator uninitialized, raises RuntimeError
 On other errors, raises RuntimeError
So in summary, hasNext() checks the current state of the traversal to determine if any nodes are left to visit, without modifying anything. Errors are raised on invalid usage.
And for next():
Parameters: None
Preconditions:
 hasNext() must be True (nodes remaining)
 Functionality:
 Returns next integer node in traversal order
 Postconditions:
 Traversal state mutated
 Next integer node returned
 Error handling:
 If hasNext() is False, raises RuntimeError
 On other errors, raises RuntimeError
So next() uses the traversal state to return the next integer and update the state appropriately. Errors are given for improper use.
Problem Decomposition
Here’s how I would break down this nested iterator problem:
Implement iterator for a nested structure that flattens it into the proper sequence.
Major parts are:
 Modeling structure as tree
 DFS traversal
 Tracking traversal state
 Iterator interface
Breakdown:
 Build tree from input
 DFS traversal logic
 Manage node visited state
 Check unvisited nodes
 Get next node
 Lazy traversal
Generalized tasks:
 Tree construction
 DFS implementation
 Traversal state tracking
 Iterator interface
 Lazy evaluation
Abstractions look good.
Names are descriptive.
Order:
 Build tree
 DFS traversal
 Manage state
 Implement iterator
 Control laziness
The key is representing as a tree, traversing with DFS, tracking state, implementing iterator on top, and controlling laziness. The breakdown identifies reusable components well.
From Brute Force to Optimal Solution
Here is a brute force approach and optimizations for the nested iterator:
Brute force:
 Flatten entire structure into one array upfront
 Iterate through array sequentially
This requires O(n) time and O(n) space to flatten initially.
Optimizations:
 Recursive flattening:
 Flatten on demand for accessed nodes using recursion.
 Avoid flattening everything upfront.
 Depthfirst ordering:
 Traverse in DFS order to generate correct sequence.
 Lazy generation:
 Only flatten when nodes accessed.
 Don’t compute nodes until needed.
 Caching:
 Memoize already visited nodes.
 Avoid reflattening seen subtrees.
 Mutable state:
 Maintain traversal state instead of flattening.
 More efficient to track position.
These optimizations improve the complexity from O(n) time and space to O(1) time per operation and O(h) recursive space for tree height h.
The key changes are: recursive ondemand flattening using DFS order, caching for efficiency, and representing state mutably rather than materializing copies.
Code Explanation and Design Decisions
Here is an analysis of the logic behind this nested iterator code:
The tree and traversal state variables represent the structure and current position.
The while loop iterates over unvisited nodes remaining. This traverses the tree.
The if condition checks node type to process correctly. This handles multiple node types.
The traversal state is mutated to track visited nodes. This maintains position.
The invariant is the traversal state accurately reflects DFS order position. This enables correct hasNext() and next() logic.
The returned array contains nodes in DFS sequence. This fulfills the flattening through traversal requirement.
Overall, the logic models the structure as a tree, leverages traversal state to flatten on demand in DFS order, handles multiple node types properly, and returns the final flattened sequence  satisfying the core requirements.
Coding Constructs
Here is an analysis of the highlevel logic of this nested iterator code:
It uses tree traversal algorithms to visit nodes in optimal order.
I would explain the code carefully traverses a tree structure to flatten it out in the right sequence.
The core logic includes recursion, conditionals, data structures, and loops.
The algorithm uses depthfirst search to incrementally flatten the tree on demand when accessed.
The key steps are building the tree, tracking traversal state, recursively flattening, and accessing in DFS order. This implements lazy tree traversal.
The overall strategy is lazy depthfirst traversal driven by the iterator interface. It encapsulates traversal state.
Overall, this analysis extracts the highlevel semantics around iterative tree traversal, lazy evaluation via state tracking, and encapsulation of the traversal details from the external interface. The focus is on the concepts over language specifics.
Language Agnostic Coding Drills
Here is a decomposition of the key coding concepts in this nested iterator problem:
Easy
 Array/list management  Adding elements, indexing
 Conditionals  Branching logic based on node types
 Looping  Iterating over elements
Medium
 Recursion  DFS traversal, unwinding call stack
 Tree structures  Modeling nesting as tree
 Graph algorithms  DFS traversal order
Advanced
 Lazy evaluation  On demand generation of nodes
 Mutable state  Tracking position via traversal state
 Encapsulation  Hiding traversal details behind iterator interface
 Amortized analysis  Understanding traversal costs amortized
The overall problem solving approach is:
 Model nested structure as tree
 Implement recursive DFS traversal
 Track traversal state
 Check state for hasNext() and next()
 Traverse nodes lazily on access
 Return flattened sequence
Each concept builds on the last to construct an efficient lazy depthfirst traversal exposed through the iterator interface. Mastering these coding fundamentals enables solving complex problems like this.
Targeted Drills in Python
Here are Python coding drills for each concept:
Basic
Array manipulation:


Conditionals:


Looping:


Intermediate
Recursion:


Tree structure:


DFS traversal:


Advanced
Lazy evaluation:


Encapsulation:


These drills can be integrated in order to build the nested iterator.
Q&A
Similar Problems
Here are 10 related LeetCode problems that involve similar concepts:
Binary Tree Inorder Traversal (94)  Inorder traversal of tree, similar iterator design.
Flatten Nested List Iterator (341)  Very similar nested structure flattening problem.
Binary Tree Level Order Traversal (102)  Tree traversal in specific ordering.
Binary Tree Zigzag Level Order Traversal (103)  Zigzag tree traversal order.
Binary Tree Right Side View (199)  Iterative tree traversal, tracking state.
Lowest Common Ancestor of Deepest Leaves (1123)  Tree traversal calculating metadata.
Maximum Depth of Nary Tree (559)  Tree traversal calculating depth.
Maximum Width of Binary Tree (662)  Calculating tree metadata through traversal.
Count Univalue Subtrees (250)  Recursive traversal calculating subtree counts.
Sum of Root To Leaf Binary Numbers (1022)  Recursive tree traversal and calculation.
Common themes are modeling trees, iterative and recursive traversals, tracking state, and encapsulating traversal details  all relevant techniques.