#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(); i ++ ){
        t += a[i] * b;
        c.push_back(t % 10);
        t /= 10;
    }
    if(t) c.push_back(t);

    while(!c.back() && c.size() > 1) c.pop_back();

    return c;
}

int main(){
    string x;
    int b;
    cin >> x >> b;
    vector<int> a;
    for(int i = x.size() - 1; i >= 0; i -- ) a.push_back(x[i] - '0');

    vector<int> c = mul(a, b);

    for(int i = c.size() - 1; i >= 0; i -- ) printf("%d", c[i]);

    return 0;
}

将mul函数中的for和if整合到一起

#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;
    }
    // if(t) c.push_back(t);

    while(!c.back() && c.size() > 1) c.pop_back();

    return c;
}

int main(){
    string x;
    int b;
    cin >> x >> b;
    vector<int> a;
    for(int i = x.size() - 1; i >= 0; i -- ) a.push_back(x[i] - '0');

    vector<int> c = mul(a, b);

    for(int i = c.size() - 1; i >= 0; i -- ) printf("%d", c[i]);

    return 0;
}