python/TangDou/LuoGuBook/L8_6.cpp

#include <bits/stdc++.h>

using namespace std;
int n;

vector<int> add(vector<int> &A, vector<int> &B) {
    if (A.size() < B.size()) return add(B, A);
    vector<int> C;
    int t = 0;
    for (int i = 0; i < A.size(); i++) {
        t += A[i];
        if (i < B.size()) t += B[i];
        C.push_back(t % 10);
        t /= 10;
    }
    if (t) C.push_back(t);
    return C;
}

vector<int> mul(vector<int> &A, int b) {
    vector<int> C;
    int t = 0;
    for (int i = 0; i < A.size() || t; i++) {
        if (i < A.size()) t += A[i] * b;
        C.push_back(t % 10);
        t /= 10;
    }
    while (C.size() > 1 && C.back() == 0) C.pop_back();
    return C;
}

int main() {
    cin >> n;
    vector<int> A, S;
    //初始化
    A.push_back(1);
    S.push_back(1);
    for (int i = 2; i <= n; i++) {
        A = mul(A, i);
        S = add(A, S);
    }
    for (int i = S.size() - 1; i >= 0; i--)printf("%d", S[i]);
    return 0;
}