Leetcode 158. 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.

思路：157的followup，需要用一个buf来存储上一次read4读入的字符，bufptr记录上一次读到的位置，bufsize记录上一次read4读入buf的大小，通过这三个变量可以在多次读入的时候知道每次该从上一次的哪里继续读入字符，以及何时再调用read4获取下一批字符。

int remainSize = 0;
int remainIndex = 0;
char[] remainBuf = new char[4];

public int read(char[] buf, int n) {
    if (n <= 0) {
        return 0;
    }

    int index = 0;
    while (index < n) {
        while (remainIndex < remainSize) {
            buf[index++] = remainBuf[remainIndex++];
        }

        if (remainIndex >= remainSize) {
            remainIndex = 0;
            remainSize = read4(remainBuf);
        }

        if (remainSize == 0) {
            break;
        }
    }

    return index;
}