157. Read N Characters Given Read4

Description

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

Example 1:

Input: buf ="abc", n = 4
Output: "abc"
Explanation: The actual number of characters read is 3, which is "abc".

Example 2:

Input: buf = "abcde", n = 5
Output: "abcde"

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

Solution

考虑下面两种情况：

• buf不够长
• file不够长

/* 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[] tmp = new char[4];
        int i = 0;
        
        while (i < n) {
            int count = read4(tmp);
            
            for (int k = 0; k < count && i < n; ++k) {
                buf[i++] = tmp[k];
            }
            
            if (count < 4) {    // file ended
                break;
            }
        }
        
        return i;
    }
}