#include <bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin >> n;
    int c1[50001] = {0};
    int lc = 1;
    c1[0] = 1;
    for(int i = 2; i <= n; i++){
        int x = 0;
        for(int j = 0; j < lc; j++){
            int sum = c1[j] * i + x;
            c1[j] = sum % 10;
            x = sum / 10;
        }
        while(x > 0){
            c1[lc] = x % 10;
            x /= 10;
            lc++;
        }
    }
    for(int i = lc - 1; i >= 0; i--){
        cout << c1[i];
    }
    return 0;
}

正确方法：

#include<bits/stdc++.h>
using namespace std;
int main()
{
    long long a;
    cin>>a;
    for(int i=a-1;i>0;i--)a*=i;
    cout<<a;
}