157 Read N Characters Given Read4

The 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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.

Note: Thereadfunction will only be called once for each test case.

The Idea: Buffer in 4 characters at a time until we either run out of characters to read, or the amount we read exceeds n. Modify the nth buffer to be the empty character for any offset issues.

Complexity: O(n/4) time and O(1) space

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


class Solution {
public:
    /**
    * @param buf Destination buffer
    * @param n Maximum number of characters to read
    * @return The number of characters read
    */
    int read(char *buf, int n) {
        int total_read = 0;
        int cur_read = 0;
        do {
            cur_read = read4(buf + total_read);
            total_read += cur_read;
        } while (cur_read == 4 && total_read < n);
        buf[n] = '\0';
        return total_read;
    }
};