题解 | #N的阶乘#

//推荐博客https://www.cnblogs.com/wuqianling/p/5387099.html，讲的很好
//字符串大数乘法，加法有些笛卡尔积的思想
#include "stdio.h"
#include "string"
#define MAX 4010
using namespace std;
int MulString(char sum[],string buf2){
    string buf1 = sum;
    int num1[MAX];int num2[MAX];int num3[MAX*2]={0};
    int len1 = buf1.size();
    int len2 = buf2.size();
    for (int i = len1-1, j = 0; i >= 0; --i) {
        num1[j++] = buf1[i] - '0';
    }
    for (int i = len2-1,j = 0; i >= 0; --i) {
        num2[j++] = buf2[i] - '0';
    }
    for (int i = 0; i < len1; ++i) {
        for (int j = 0; j < len2; ++j) {
            num3[i+j] = num3[i+j]+num1[i]*num2[j];
        }
    }
    for (int i = 0; i < len1+len2+1; ++i) {//依次进位
        if (num3[i] > 9){
            num3[i+1] = num3[i+1] + num3[i]/10;
            num3[i] = num3[i] % 10;
        }
    }
    int len3;
    for (len3 = MAX*2-1; num3[len3] == 0 && len3 >= 0 ; --len3);
    ++len3;
    for (int i = 0; i < len3; ++i) {
        sum[len3-i-1] = num3[i] + '0';
    }
    return len3;
}

int main(){
    int N;
    while (scanf("%d",&N)!=EOF){
        char sum[MAX*2]={};
        sum[0] = '1';
        int len;
        for (int i = 2; i <= N; ++i) {
            len = MulString(sum,to_string(i));
        }
        for (int i = 0; i < len ; ++i) {
            printf("%c",sum[i]);
            sum[i] = '0';
        }
        printf("\n");
    }
}