题解 | #N的阶乘#

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
//大数加法
string addString(string a, string b) {
    int carry = 0;
    string res;
    int i = a.size() - 1;
    int j = b.size() - 1;
    while (i >= 0 && j >= 0) {
        int num = carry + a[i] - '0' + b[j] - '0';
        res += num % 10 + '0';
        carry = num / 10;
        i--;
        j--;
    }
    while (i >= 0) {
        int num = carry + a[i] - '0';
        res += num % 10 + '0';
        carry = num / 10;
        i--;
    }
    while (j >= 0) {
        int num = carry + b[j] - '0';
        res += num % 10 + '0';
        carry = num / 10;
        j--;
    }
    if (carry > 0)
        res += to_string(carry);
    reverse(res.begin(), res.end());
    return res;
}
//大数乘法(x<=10)
string mulString(string a, int x) {
    int carry = 0;
    string res;
    for (int i = a.size() - 1; i >= 0; i--) {
        int num = carry + (a[i] - '0') * x;
        res += num % 10 + '0';
        carry = num / 10;
    }
    if (carry > 0)
        res += to_string(carry);
    reverse(res.begin(), res.end());
    return res;
}
//组合大数乘法
string MulString(string a, string b) {
    string res;
    for (int i = 0; i < b.size(); i++) {
        int x = b[b.size() - 1 - i] - '0';
        string num = mulString(a, x);
        for (int j = 0; j < i; j++)
            num = mulString(num, 10);
        res = addString(res, num);
    }
    return res;
}
int main() {
    int n;
    while (cin >> n) {
        string res = "1";
        for (int i = 2; i <= n; i++) {
            res = MulString(res, to_string(i));
        }
        cout << res << endl;
    }
    return 0;
}