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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）每次从一个文件读取4个字符。

返回值是读取的实际字符数。例如，如果文件中只剩下3个字符，则返回3。

通过使用read4 API，实现从文件中读取n个字符的int read（char*buf，int n）函数。

注：

读取函数可以被多次调用。

 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 }