没毛病

#include <bits/stdc++.h>
using namespace std;
string s1,s2;
int a[10005],b[10005],c[10005],id,len;
int main(){
    cin >> s1 >> s2;
    int n = s1.size();
    int m = s2.size();
    for(int i = n-1;i >= 0;i--){
        a[id] = s1[i] - '0';
        id++;
    }
    id = 0;
    for(int i = m-1;i >= 0;i--){
        b[id] = s2[i] - '0';
        id++;
    }
    for(int i = 0;i < n;i++){
        for(int j = 0;j < m;j++){
            c[i+j] += a[i] * b[j];
            c[i+j+1] += c[i + j]/10;
            c[i+j]%=10;
        }
    }
    len = n + m;
    while(c[len] == 0 && len > 0) len--;
    for(int i = len;i >= 0;i--){
        cout << c[i];
    }
    return 0;
}