题解 | #N的阶乘#

大整数乘法，这无脑了

#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
using namespace std;
string cal(string num, int n) {
    string val = to_string(n), ansstr = "";
    int len = num.size() * val.size() + 1;
    int* ans = new int[len + 1];
    for (int i = 0; i <= len; ++i)
        ans[i] = 0;
    reverse(num.begin(), num.end());
    reverse(val.begin(), val.end());
    for (int i = 0; i < num.size(); ++i) {
        for (int j = 0; j < val.size(); ++j) {
            ans[i + j] += (num[i] - '0') * (val[j] - '0');
        }
    }
    for (int i = 0; i < len; ++i) {
        ansstr = to_string(ans[i] % 10) + ansstr;
        ans[i + 1] += ans[i] / 10;
    }
    ansstr = to_string(ans[len]) + ansstr;
    int pos = ansstr.find_first_not_of('0');
    if (pos == string::npos)
        return 0;
    else
        return ansstr.substr(pos);
}
int main()
{
    int n;
    string ans = "";
    while (cin >> n) {
        ans = "1";
        for (int i = 2; i <= n; ++i)
            ans = cal(ans, i);
        cout << ans << endl;
    }
}