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

vector<int> A,B;

vector<int> mul(vector<int> A, vector<int> B)
{
    vector<int> C(A.size()+B.size()+3, 0);
    //对位相乘
    for(int i = 0; i < A.size(); i ++)
        for(int j = 0; j < B.size(); j ++)
            C[i+j] += A[i]*B[j]; //+=

    //进位
    for(int i = 0; i < C.size(); i ++)
    {
        C[i+1] += C[i]/10;
        C[i] %= 10;
    }
    //除去前导零
    while(C.size()>1 && C.back() == 0)
        C.pop_back();
    return C;
}
int main()
{
    string a,b;
    cin >> a >> b;
    for(int i = a.size()-1; i >= 0; i--)
        A.push_back(a[i]-'0');
    for(int i = b.size()-1; i >= 0; i--)
        B.push_back(b[i]-'0');

    vector<int> C = mul(A, B);
    for(int i = C.size()-1; i >= 0; i--)
        cout << C[i];
    return 0;
}

来自站内大佬的图：