add_string.cpp

#include <bits/stdc++.h>
using namespace std;

#pragma GCC target   ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("-ffloat-store")
#pragma GCC optimize ("Ofast")

static auto _ = [] () {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    return 0;
}();

#define fr(i, len) for(int i=0; i < len; i++)

#define YES cout<<"Yes \n";
#define NOO cout<<"No  \n";
#define nl cout<<"\n";

#define MAX_SIZE 10000
#define nptr nullptr

typedef stringstream strgm;
typedef long long int ll;
typedef const int c_int;
typedef unsigned unsg;
typedef double dbl;

typedef vector<vector<string>> vvstr;
typedef vector<vector<bool>> vvbl;
typedef vector<vector<int>> vvint;
typedef vector<vector<ll>> vvll;

typedef vector<string> vstr;
typedef vector<int> vint;
typedef vector<bool> vbl;
typedef vector<ll> vll;

typedef stack<string> sstr;
typedef stack<bool> sbl;
typedef stack<int> sint;
typedef stack<ll> sll;

typedef queue<string> qstr;
typedef queue<bool> qbl;
typedef queue<int> qint;
typedef queue<ll> qll;

c_int MOD=1e9 + 7;
c_int Mx_row=100;
c_int Mx_col=100;
int size_stack=0;
int InvrsnCnt=0;
int size_arr=0;
int size_ll=0;
int top=-1;

string main(){
    string s1,s2;
    cin>>s1>>s2;

    int i = s1.size()-1;
    int j = s2.size()-1;

    int carry = 0;
    string ans = "";

    while(i >= 0 || j >= 0 || carry){
        int sum = carry;
        if(i >= 0) sum += s1[i--] - '0';
        if(j >= 0) sum += s2[j--] - '0';

        carry = sum / 10;
        
        int temp = sum % 10;
        ans.push_back(temp + '0');
    }

    reverse(ans.begin(),ans.end());
    return ans;
}