157 Read N Characters Given Read4

来源：Leetcode

原帖：https://leetcode.com/problems/read-n-characters-given-read4/

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

代码：

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