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.

API：int Read 4（char *BUF）每次從一個文件讀取4個字符。

返回值是讀取的實際字符數。例如，如果文件中只剩下3個字符，則返回3。

通過使用read4 API，實現從文件中讀取n個字符的int read（char*buf，int n）函數。

註：

讀取函數可以被多次調用。

 1 class Solution {
 2     var readPos:Int = 0
 3     var writePos:Int = 0
 4     var buff:[Character] = [Character](repeating:"
 
 ",count:4)
 5     func read(_ buf:[Character],_ n:Int) -> Int {
 6         var i:Int = 0
 7          while (i < n && (readPos < writePos || (readPos = 0) < (writePos = read4(buff))))
 8         {
 9             buf[i] = buff[readPos]
10             i += 1
11             readPos += 1
 

12         }
13         return i
14     }
15 }

[Swift]LeetCode158. 用Read4來讀取N個字符II $ Read N Characters Given Read4 II