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.

一刷
题解:与157不同的是,函数可以被多次调用,比如read(buf, 1), 但是我们访问read4,如果函数没有结束,一定读得了4个char,但是只返回一个。
buffPtr: buffer pointer
buffCnt: buffer counter, 保存一次调用read4读到的字符数目
直到buffPtr reach buffCnt的时候,置为0,准备调用下一次read4

/* 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 buffPtr = 0;//buffPointer
    private int buffCnt = 0;//buffCounter
    private char[] buff = new char[4];
    
    
    public int read(char[] buf, int n) {
        int ptr = 0;
        while(ptr<n){
            if(buffPtr == 0){
                buffCnt = read4(buff);
            }
            if(buffCnt == 0) break;
            while(ptr<n && buffPtr < buffCnt){//if buffPtr reach the buffCnt, set to zero and ready
                buf[ptr] = buff[buffPtr];
                ptr++;
                buffPtr++;
            }
            if(buffPtr>=buffCnt) buffPtr = 0;
        }
        return ptr;
    }
}

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