vector求助大佬

#include<iostream>
#include<vector>
#include<string>
using namespace std;
string a, b;
vector<int> mul(vector<int> A, vector<int> B)
{
    int num = a.size() + b.size() - 1;
    vector<int> C(num+1);
    for (int i = 0; i < num; i++)
    {
        for (int j = 0; j <= i; j++)
        {
            C[i] += A[j] * B[i - j];
        }
        if (C[i] >= 10)
        {
            C[i + 1] += C[i] / 10;
            C[i] %= 10;
        }
    }
    while (C.back() == 0)
        C.pop_back();
    return C;
}
int main()
{
    cin >> a;
    cin >> b;
    vector<int> A;
    vector<int> B;
    for (int i = a.size() - 1; i--; i >= 0)
        A.push_back(a[i]-'0');
    for (int i = b.size() - 1; i--; i >= 0)
        B.push_back(b[i]-'0');
    for (int i = A.size(); i < a.size() + b.size() - 1; i++)
        A.push_back(0);
    for (int i = B.size(); i < a.size() + b.size() - 1; i++)
        B.push_back(0);
    auto C = mul(A, B);
    for (int i = C.size() - 1; i--; i >= 0)
        cout << C[i];
    return 0;
}