清华大学机试 N的阶乘 *大数低精度乘法问题

#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
using namespace std;

const int maxn = 100000;
struct bign {
    int len;
    int num[maxn];
    bign() {
        fill(num, num + maxn, 0);
        len = 0;
    }
};

bign trans(int n) {
    bign a;
    while (n!=0){
        a.num[a.len++] = n % 10;
        n /= 10;
    }
    return a;
}

bign multi(bign a, int n) {
    int carry = 0;
    for (int i = 0; i < a.len; i++) {
        int temp = carry + a.num[i] * n;
        a.num[i] = temp % 10;
        carry = temp / 10;
    }
    while (carry != 0) {
        a.num[a.len++] = carry % 10;
        carry /= 10;
    }
    return a;
}

string bign2string(bign a) {
    string s = "";
    for (int i = a.len - 1; i >= 0; i--) {
        s += char(a.num[i] + ‘0‘);
    }
    return s;
}

int main() {
    int n;
    while (cin>>n){
        if (n == 0) {
            cout << 0 << endl;
            continue;
        }
        bign a = trans(n);
        for (int i = n - 1; i >= 2; i--) {
            a=multi(a, i);
        }
        cout << bign2string(a) << endl;
    }
    return 0;
}