方法一

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

int main(){
    string a;
    int b;
    cin >> a >> b;
    reverse(a.begin(), a.end());
    int cnt = 0;
    string res = "";
    for(int i = 0; i < a.size(); i ++ ){
        int num = (a[i] - '0') * b + cnt;
        if(num >= 10){
            cnt = num / 10;
            num %= 10;
        }else   cnt = 0;
        res += (num + '0');
    }
    while(cnt){
        res += (cnt % 10 + '0');
        cnt /= 10;
    }
    reverse(res.begin(), res.end());
    int flag = 1;
    for(int i = 0; i < res.size(); i ++ ){
        if(flag){
            if(res[i] != '0'){
                cout << res[i];
                flag = 0;
            }
        }else   cout << res[i];
    }
    if(flag == 1)   cout << "0" << endl;
    return 0;
}

方法二（精简版）

#include <iostream>
#include <vector>
using namespace std;
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(){
    string a;
    int b;  cin >> a >> b;
    vector<int> A;
    for(int i = a.size() - 1; i >= 0; i -- )    A.push_back(a[i] - '0');
    vector<int> C = mul(A, b);
    for(int i = C.size() - 1; i >= 0; i -- )    cout << C[i];
    return 0;
}