问题描述：
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.
Example 1:
Input: buf = "abc", n = 4 Output: "abc" Explanation: The actual number of characters read is 3, which is "abc".
Example 2:
Input: buf = "abcde", n = 5 Output: "abcde"
Note:
The read function will only be called once for each test case.
解题思路：
讲真这道题我读题的时候就一脸懵逼。
他给的例子中描述了当字符串长度<=n的情况。
但是也有字符串长度>n的情况
我们还要考虑空字符串的情况（这些都是我试出来的ಠ_ಠ
我们有total来存储当前读到的字符个数，用num来存储当前用read4读到的字符个数。
如果num = 0 说明字符个数小于n这时候我们可以直接返回num。
需要注意的是：要移动指针。
代码：
// 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) { int total = 0; while(total < n){ int num = read4(buf); total += num; buf += num; if(num == 0) return total; } return total > n ? n : total; } };