Read N Characters Given Read4

Too many down votes. This is a waste of time. Spend very less time.

This problem can be solved by calling read4 in a loop, until we have read n characters or we have reached the end of the file.

Here is the implementation in C++:

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// Forward declaration of the read4 API.
int read4(char *buf4);

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n Number of characters to read
     * @return The number of actual characters read
     */
    int read(char *buf, int n) {
        int total = 0;
        while (total < n) {
            char buf4[4];
            int curr = min(read4(buf4), n - total);
            for (int i = 0; i < curr; i++) {
                buf[total++] = buf4[i];
            }
            // if we read less than 4 characters, we've reached the end of the file
            if (curr < 4) break;
        }
        return total;
    }
};

In this code:

We call read4 in a loop until we have read n characters or we reach the end of the file.
We use min(read4(buf4), n - total) to ensure we do not read more than n characters.
We use a for loop to add the characters read to the destination buffer buf.
If we read less than 4 characters from read4, it means we have reached the end of the file, so we break out of the loop.
Finally, we return the total number of characters read, which is total.

This solution ensures that we read exactly n characters or the total characters in the file if n is greater than the file length. The time complexity is O(n), as we read n characters.

In Python:

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"""
The read4 API is already defined for you.

    @param buf4, a list of characters
    @return an integer
    def read4(buf4):

# Below is an example of how the read4 API can be called.
file = File("abcdefghijk") # File is "abcdefghijk", initially file pointer (fp) points to 'a'
buf4 = [' '] * 4 # Create buffer with enough space to store characters
read4(buf4) # read4 returns 4. Now buf = ['a','b','c','d'], fp points to 'e'
read4(buf4) # read4 returns 4. Now buf = ['e','f','g','h'], fp points to 'i'
read4(buf4) # read4 returns 3. Now buf = ['i','j','k',...], fp points to end of file
"""

class Solution:        
    def read(self, buf: list, n: int) -> int:
        total = 0
        while total < n:
            buf4 = [' '] * 4
            current = min(read4(buf4), n - total)
            for i in range(current):
                buf[total] = buf4[i]
                total += 1
            if current < 4:
                break
        return total

The read4 function is defined as a stub because the actual implementation is provided by the system. The read function works the same as the C++ code. It continuously calls read4 to read the characters into buf4 and then copies the characters to buf until total reaches n or the file ends. Finally, it returns the total number of characters read.