C语言 文件输入输出

输入：freopen(“xxx.txt”,“r”,stdin);
输出：freopen(“xxx.txt”,“w”,stdout);

#include<cstdio>
int main()
{
    int n;
    freopen("123.txt","r",stdin);
    scanf("%d",&n);
    freopen("321.txt","w",stdout);
    printf("%d",n);
}