Valid Palindrome
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Identifying Problem Isomorphism
“Valid Palindrome” can be mapped to the following problems:
- “Palindrome Number” (#9) on LeetCode
- “Valid Palindrome II” (#680) on LeetCode
Reasoning:
These problems involve checking if a sequence, string or number, is a palindrome.
“Palindrome Number” is the simplest problem, where you are asked to determine if an integer is a palindrome. It’s simpler because you don’t have to handle the additional complexity of non-alphanumeric characters or case sensitivity.
“Valid Palindrome II” is a more complex problem, as it allows for the removal of one character in the string to see if a valid palindrome can be created. It builds on the concept of checking for a palindrome and adds an additional layer of complexity in that you may modify the sequence slightly to form a valid palindrome.
While these problems are not exact isomorphs of “Valid Palindrome,” they all revolve around the same core concept of checking if a sequence is a palindrome.
10 Prerequisite LeetCode Problems
“Valid Palindrome” (LeetCode 125) is a straightforward problem and is a good starting point for learning about string manipulation and the two-pointer technique. It requires very little in terms of prerequisite knowledge or skills.
Here are a few simpler problems, focusing on string manipulation:
- “344. Reverse String”: This problem is even simpler than “Valid Palindrome”, asking you to reverse a string, which can be done with a straightforward loop.
- “709. To Lower Case”: This problem requires you to change all characters in a string to lowercase.
- “771. Jewels and Stones”: This problem requires counting the occurrences of characters from one string in another string.
- “804. Unique Morse Code Words”: This problem involves converting strings to Morse code and counting unique results.
- “806. Number of Lines To Write String”: This problem involves calculating the number of lines needed to write a string given constraints on line length.
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Problem Classification
String Manipulation: The task involves working with a string input,
s
, and traversing it to validate a condition, which is a fundamental operation of string manipulation.Two Pointers Technique: The algorithm uses two pointers,
i
andj
, which traverse the string from both ends towards the center. The two-pointer technique is a common strategy for solving array and string-related problems, particularly when the order or sequence of the elements plays a crucial role in the problem.Palindrome Check: The main goal of this problem is to determine whether a given string is a palindrome or not, ignoring spaces, punctuation and case sensitivity. Palindromes are a common theme in problems involving strings.
Character Properties: The problem involves checking whether each character is alphanumeric. This falls under understanding and manipulating the properties of characters in a string.
These categories can help to find similar problems for practice or to identify appropriate problem-solving techniques and algorithms.
Language Agnostic Coding Drills
Variable Assignments:
- Declare variables of various types (integers, floats, strings, boolean etc.)
- Perform various operations on variables (addition, multiplication, division, subtraction)
- Swap the values of two variables without using a third variable
Control Structures:
- Write a code snippet to check if a number is even or odd (if-else statement)
- Write a code snippet to find the maximum of three numbers (nested if-else)
Loops:
- Print numbers from 1 to 10 using a loop
- Write a loop to calculate the factorial of a number
- Write a loop to print a Fibonacci series up to a certain number
Functions:
- Write a function that takes two numbers as parameters and returns their sum
- Write a recursive function to calculate the factorial of a number
- Write a function to check if a string is palindrome
Data Structures:
- Create a list/array and perform basic operations (insert, delete, access)
- Create a dictionary/hashmap and perform basic operations (insert, delete, access)
- Implement a stack or a queue and demonstrate the push/pop or enqueue/dequeue operations
Sorting and Searching Algorithms:
- Implement a linear search algorithm
- Implement a binary search algorithm
- Implement basic sorting algorithms like bubble sort, selection sort, or insertion sort
Classes and Objects (for OOP languages):
- Create a basic class with a constructor, some fields, and methods
- Instantiate objects of the class and manipulate fields and methods
- Write a class that inherits from another class and demonstrate method overriding
Targeted Drills in Python
- Working with Strings: Understand the basics of how strings work in Python, how to access characters in a string through indexing, and how to determine the length of a string. Implement simple operations like lower-casing all characters in a string.
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- String Traversal: Understand how to traverse a string using loops, especially focusing on traversing from both ends towards the middle.
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- Character Classification: Understand how to classify characters in Python, especially distinguishing alphanumeric characters (letters and digits) from other characters.
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- Comparing Characters: Understand how to compare characters in Python, and how to handle comparison of characters with different cases.
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- Boolean Operations: Understanding boolean operations is crucial for the conditional statements in this problem. Practice using boolean logic in Python and applying it in conditional statements.
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Once comfortable with these smaller coding exercises, one can start to combine these concepts and work towards the final solution, where you traverse the string from both ends, ignoring non-alphanumeric characters and comparing the alphanumeric ones. If at any point the characters do not match, you return False
, and if you manage to traverse the whole string this way, you return True
.
My Notes
title: Valid Palindrome excerpt: This covers the basic building blocks Alphanumeric Check and Two Pointers Moving Towards Each Other. tags: palindrome alphanumeric-check two-pointers-moving-towards-each-other
Given a string s, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
Example 1:
Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.
Example 2:
Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.
Constraints
- 1 <= s.length <= 2 * 10^5
- s consists only of printable ASCII characters.
Implementation
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Building Blocks
- Alphanumeric Check
- Two Pointers Moving Towards Each Other