Read N Characters Given Read4 - Leetcode Solution

The Read N Characters Given Read4 problem on LeetCode is a popular problem which tests the ability of a programmer to read n characters from a file using the read4 API.

The problem statement on LeetCode says:

The API read4 reads 4 consecutive characters from a file and returns the number of characters read. The API can be called multiple times.

Implement the method read(char *buf, int n) which reads n characters from the file using read4 API and stores it in the buffer buf. The function should return the number of characters actually read.

Note: The read function may be called multiple times.

Here, the API read4 is already provided for us, which reads 4 consecutive characters from a file and returns the number of characters read. Our task is to implement the read() function which reads n characters from the file using the read4() function.

One important thing to note here is that the read4() API reads 4 characters at a time, so if we need to read more than 4 characters, we need to call the read4() API multiple times. We also need to take care of cases where the number of characters to be read is not a multiple of 4.

Now, let's discuss the steps to solve this problem:

Step 1: Initialize two variables - totalCharsRead and charsRead to keep track of total characters read and number of characters read in each iteration respectively.

Step 2: Create a while loop which runs until we have read n characters from the file. In each iteration, we read 4 characters from the file using the read4() API.

Step 3: Check if the read4() API has returned any characters. If it has not, we break from the loop as we have nothing more to read.

Step 4: Copy the characters returned by the read4() function to buffer buf and increment the totalCharsRead by the number of characters read in the current iteration.

Step 5: In case the number of characters read by read4() is greater than the number of characters we need to read, we need to truncate the buffer to the required length.

Here is the implementation of the solution in C++:

int read4(char *buf);

class Solution {
public:
    int read(char *buf, int n) {
        int totalCharsRead = 0;
        int charsRead = 0;
        char tempBuf[4];
        while (totalCharsRead < n) {
            charsRead = read4(tempBuf);
            if (charsRead == 0) {
                break;
            }
            for (int i = 0; i < charsRead && totalCharsRead < n; i++) {
                buf[totalCharsRead++] = tempBuf[i];
            }
        }
        return totalCharsRead;
    }
};

In the above code, we have created a class called Solution which contains a read() function that implements the solution discussed above.

We have initialized two variables - totalCharsRead and charsRead to 0. We have also created a temporary buffer of size 4 where we will store the characters read from the file using read4() function.

In the while loop, we call the read4() function and store the number of characters read in the charsRead variable. We then check if the read4() function has returned any characters.

If it has not returned any characters, we break from the loop as we have nothing more to read. Otherwise, we copy the characters returned by the read4() function to buffer buf and increment the totalCharsRead variable by the number of characters read in the current iteration.

Finally, we return the totalCharsRead variable which contains the number of characters read from the file.

In conclusion, the read N Characters Given Read4 problem on LeetCode is a popular problem which requires a good understanding of the read4() function. The solution involves calling the read4() function in a loop until we have read n characters from the file. We also need to take care of cases where the number of characters to be read is not a multiple of 4.

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