157. Read N Characters Given Read4

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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.

Note:
Thereadfunction will only be called once for each test case.

    int read(char *buf, int n) {
        int result = 0;
        while (n > 0) {
            int cur = min(read4(buf), n);
            result += cur;
            buf += 4;
            n -= 4;
        }
        return result;
    }

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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.

Note:
Thereadfunction may be called multiple times.

S: external buffer vs. internal buffer

当上一次read4()读了四个数，文件系统指针已经移动到了四个位置以后, 当前用到k < 4时， 需要用internal buffer存储k以后的信息，下次从k + 1开始读而不是四个位置之后

int read4(char *buf);
class Solution {
public:
    int read(char *buf, int n) {
        int idx = 0;
        while(idx < n) {
            if (buffPtr == buffCnt) {  //already wrote all cached value, start another read4
                buffPtr = 0;
                buffCnt = read4(buf4);
                if (buffCnt == 0) break;
            }
            buf[idx++] = buf4[buffPtr++];
        }
        return idx;
    }

private:
    char buf4[4];
    int buffPtr = 0;
    int buffCnt = 0;
};