[leetcode] 157. Read N Characters Given Read4 解题报告

题目链接：https://leetcode.com/problems/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 the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file.

Note:
The read function will only be called once for each test case.

思路：来不及解释了，快上车。

// Forward declaration of the read4 API.
int read4(char *buf);

class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    int read(char *buf, int n) {
        int readLen = 0, offset = 0;
        char buffer[4];
        while(offset<n && (readLen=read4(buffer)) >0)
        {
            int wLen = min(n-offset, readLen);
            memcpy(buf+offset, buffer, wLen);
            offset += wLen;
        }
        return offset;
    }
};