Leetcode - Read N Characters Given Read4 II - Call Multiple Times

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 may be called multiple times.

[分析]
read方法会被调用多次的难点在于如何处理read4读取出来的内容有剩余的情况？假设文件中有4个字符，我们调用4次read（1）,通过一次read4把4个字符全部读取，如何让每次read只读1个字符？ 方案就是读完后做标记，read4读出来的相当于文件到输出的缓冲区，标记缓冲区当前位置以及字符总数，每次read从缓冲区当前位置开始读取所需字符数。
参考：
https://leetcode.com/discuss/21219/a-simple-java-code
http://www.danielbit.com/blog/puzzle/leetcode/leetcode-read-n-characters-given-read4-ii

/* 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
     */
    private int bufferOffset = 0;
    private int bufferSize = 0;
    private char[] buffer = new char[4];
    public int read(char[] buf, int n) {
        int readBytes = 0;
        while (readBytes < n) {
            if (bufferOffset == 0) 
                bufferSize = read4(buffer);
            if (bufferSize == 0) break; // reach end of file
            while (readBytes < n && bufferOffset < bufferSize) {
                buf[readBytes++] = buffer[bufferOffset++];
            }
            if (bufferOffset == bufferSize)
                bufferOffset = 0;
        }
        return readBytes;
    }
}