string实现高精度加法，通过反转字符串使得加法过程更简单且不用考虑前导零。

#include <iostream>
using namespace std;

string rev(string a){
    string res;
    for(char c:a) res = c + res;
    return res;
}

string remove_pre0(string a){
    int i = a.size() - 1;
    while(a.size() >= 1){
        if(a[i] == '0') a = a.substr(0, a.size()-1), i = a.size() - 1;
        else break;
    }
    if(a == "") return "0";
    return a;
}
string add(string a, string b){
    int len = min(a.size(), b.size());
    string res;
    int carry = 0;
    int i = 0;
    for(i = 0; i < len; i++){
        int sum = a[i] - '0' + b[i] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);
    }
    while(i < a.size()){
        int sum = a[i++] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);       
    }
    while(i < b.size()){
        int sum = b[i++] - '0' + carry;
        carry = sum / 10;
        sum %= 10;
        res += to_string(sum);       
    }  
    if(carry) res += to_string(carry);
    remove_pre0(res);
    return res;
}

int main(){
    string a, b;
    cin>>a>>b;
    cout << rev(add(rev(a), rev(b)));
}