[leetcode]Read N Characters Given Read4

用read4实现readn...

至调用一次，感觉怎么搞都可以。。。估计这个题有II就是调用多次了。。。

感觉多次勇哥buffer存下多读的那部分就好了。。。

// 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) {
        char buffer[5];
        int cnt = 0;
        while (cnt < n) {
            int sz = read4(buffer);
            memcpy(buf + cnt, buffer, sz);
            cnt += sz;
            if (sz < 4) break;
        }
        if (cnt > n) {
            buf[n] = '';
            cnt = n;
        }
        return cnt;
    }
};