LeetCode #157 - Read N Characters Given Read4

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本文介绍了一种利用read4 API实现read函数的方法，该函数可以从文件中读取指定数量的字符。通过循环调用read4并检查返回值，确保读取过程正确处理文件末尾情况。

题目描述：

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"

// 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 count=0;
        while(true)
        {
            int x=read4(buf+count); // buf+count就表示从buf[count]处开始读取
            count+=x;
            n-=x;
            if(n<=0)
            {
                count+=n;
                break;
            }
            if(x<4) break;
        }
        return count;
    }
};