Point:

1、公式

$$ tmp = tmp + a[i] + (i < b.size() \; ? \; b[i] \; : \; 0)$$

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

// 加引用 快
vector<int> Vplus(vector<int>& a, vector<int>& b)
{
    if(a.size() < b.size()) return Vplus(b, a);
    vector<int> c;
    int t = 0;
    for(int i = 0; i < a.size(); ++ i)
    {
        t += a[i];
        if(i < b.size()) t += b[i];
        c.push_back(t % 10);
        t /= 10;
    }
    if(t) c.push_back(t);
    return c;
}

int main()
{
    string a, b;
    vector<int> A, B;
    cin >> a >> b;

    // 逆序输入
    for (int i = a.size() - 1; i >= 0; --i) A.push_back((int)(a[i] - '0'));
    for (int i = b.size() - 1; i >= 0; --i) B.push_back((int)(b[i] - '0'));
    A = Vplus(A, B);
    // 逆序输出
    for (int i = A.size() - 1; i >= 0; --i) cout << A[i];
}