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.

Solution：需先读Cache

思路：
The only thing is when you call read4() which reads 4 bytes into your buffer you might read more than you need, so you want to store those bytes in the structure, and next time you call read will start from those stored bytes, then read more from the file.

变化是这样：比如先call了n=3，然后call n=5，那么第一次就读入了4个char，第二次call应该把上一次的最后一个char拿来。也就是说要有个cache取缓存已读取的字符，然后从这个cache里面取。

Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

/* 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
     */
    char[] prevBuf = new char[4];
    int prevSize = 0;
    int prevIndex = 0;
    
    public int read(char[] buf, int n) {
        int counter = 0;
        
        while (counter < n) {
            if (prevIndex < prevSize) {
                buf[counter++] = prevBuf[prevIndex++];
            } else {
                prevSize = read4(prevBuf);
                prevIndex = 0;
                if (prevSize == 0) {
                    // no more data to consume from stream
                    break;
                }
            }
        }
        return counter;
    }
}