Read N Characters Given Read4 – LeetCode 157

Problem

Description

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 will only be called once for each test case.

Answer

Original

Code

// Forward declaration of the read4 API.
int read4(char *buf);

class Solution {
public:
    int read(char *buf, int n) {
        int res = 0;
        for (int i = 0; i <= n / 4; ++i) {
            int cur = read4(buf + res);
            if (cur == 0) break;
            res += cur;
        }
        return min(res, n);
    }
};

思路

简单的循环读取，如果提早结束了就判断并返回，在n不是4的倍数但文件大小大于n时会存在信息泄露的安全风险，不过胜在逻辑清晰。时间复杂度$O(n)$，空间复杂度$O(1)$。

Better

还没看到更好的思路。

FXTi's blog

read-n-characters-given-read4