LeetCode 157. Read N Characters Given Read4

最新推荐文章于 2020-12-20 18:55:38 发布

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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，直到读取到所需字符数或文件结束，返回实际读取的字符数。适用于文件长度可能小于、等于或大于所需读取字符数的情况。

本文转载自 : https://blog.youkuaiyun.com/xinqrs01/article/details/54093704

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.

思路：
1. 用read4读，读一次，修改一次指针，让buf指向下一个4字节开始
2. 关键是：要求读n个，file总长度可能大于等于n，或者小于n。当file总长度小于n，则返回实际读出的长度！

int read4(char *buf);

class Solution {
public:
    int read(char *buf, int n){
        int total=0; cur=0;
        while(cur==4&&total<n){
            cur=read4(buf);
            buf=buf+cur;
            total+=cur;
        }
        return min(total+cur,n);
    }
};
或者：

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