关键

1. 小端存储
2. 模拟加法，处理进位情况

c++

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
using namespace std;
const int N=1e5+10;
vector<int> A,B;

vector<int> add(vector<int>& A,vector<int>& B){
    vector<int> res;
    if(A.size()<B.size()) return add(B,A);
    int t=0;
    for(int i=0;i<A.size();i++){
        t+=A[i];
        if(i<B.size()) t+=B[i];
        res.push_back(t%10);
        t=t/10;
    }
    if(t) res.push_back(t);
    return res;
}

int main(){
    string s1,s2;
    cin>>s1>>s2;
    for(int i=s1.length()-1;i>=0;i--) A.push_back(s1[i]-'0');
    for(int i=s2.length()-1;i>=0;i--) B.push_back(s2[i]-'0');
    auto C=add(A,B);
    for(int i=C.size()-1;i>=0;i--) printf("%d",C[i]);
    printf("\n");
    return 0;
}