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

Solution：

思路： 循环read4来读4个字符并将结果复制到result buf中，直到最后一次到文件End了(read4返回count < 4)或是读完想要读的n个字符了。 考虑到最后一次时，(1)要复制的次数count = Math.min(count, n - total); //count: from read4, n - total 还需要读的字符，即读的超出了想要读的数量， 故先用read4到tmp缓存
Time Complexity: O(N) Space Complexity: O(1) 不算结果

Solution Code：

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) {
        boolean eof = false;      // end of file flag
        int total = 0;            // total bytes have read
        char[] tmp = new char[4]; // temp buffer

        while (!eof && total < n) {
            int count = read4(tmp);

            // check if it's the end of the file
            eof = count < 4;

            // get the actual count
            count = Math.min(count, n - total);

            // copy from temp buffer to buf
            for (int i = 0; i < count; i++) 
                buf[total++] = tmp[i];
            }

        return total;
    }
}