Read N Characters Given Read4

Problem

he API: int read4(char *buf) reads 4 characters at a time from a file.

The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file.

By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.

Note: The read function will only be called once for each test case.

Solution

/* The read4 API is defined in the parent class Reader4.
      int read4(char[] buf); */

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    public int read(char[] buf, int n) {
        char[] temp = new char[4];
        int index = 0;
        boolean end = false;
        while (!end && index < n) {
            int len = read4(temp);
            end = len != 4;
            int realCount = Math.min(len, n - index);
            for (int i = 0; i < realCount; i++) buf[index++] = temp[i];
        }
        return index;
    }
}

Analysis

The given method read4(char[] buf) will read some file and write the result to buf
We need to read n lines of file and save the result to buf
Hence we have a char[4] temp to pass from read4 to buf
Notice that we need to make sure we read n lines, hence we need to get realCount with smaller of len and left (n - total)