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）每次從一個檔案讀取4個字元。

返回值是讀取的實際字元數。例如，如果檔案中只剩下3個字元，則返回3。

通過使用read4 API，實現從檔案中讀取n個字元的int read（char*buf，int n）函式。

注：

讀取函式可以被多次呼叫。

 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 }