Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <vector>
#include <iostream>
#include <set>
#include <cstdio>
#include <queue>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <ctime>
#include <cmath>
#include <complex>
#include <algorithm>
#include <tuple>
#include <algorithm>
#include <limits>
#include <map>
#include <valarray>
#include <list>

using namespace std;

typedef long long ll;

int main() {
    int t;
    cin >> t;
    while (t--) {
        string s1, s2;
        cin >> s1 >> s2;
        if (s1 == "0" && s2 == "0") {
            cout << "0" << endl;
            continue;
        }
        reverse(s1.begin(), s1.end());
        reverse(s2.begin(), s2.end());
        int n = max(s1.size(), s2.size()) + 1;
        for (int i = s1.size(); i < n; i++) {
            s1 += '0';
        }
        for (int i = s2.size(); i < n; i++) {
            s2 += '0';
        }
        string res;
        int b = 0;
        for (int i = 0; i < max(s1.size(), s2.size()); i++) {
            int u = s1[i]-'0'+s2[i]-'0'+ b;
            b = (u >= 10); u %= 10;
            res += u + '0';
        }
        reverse(res.begin(), res.end());
        string res2;
        int j = 0;
        while (res[j] == '0') j++;
        for (; j < res.size(); j++) {
            res2 += res[j];
        }
        if (res2.size() > 80) {
            cout << "overflow" << endl;
        } else {
            cout << res2 << endl;
        }
    }
}

C++: #include <iostream>
using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin>>n;
    cin.ignore();
    for(int i=0; i<n; i++){
        char a[128], b[128], c[128];
        int al, bl;
        cin.getline(a, 128);
        al=cin.gcount()-1;
        cin.getline(b, 128);
        bl=cin.gcount()-1;
        int j=al, k=bl, carry=0, l=0;
        while(true){
            j--, k--;
            if(j<0||k<0) break;
            c[l]=a[j]+b[k]+carry-'0';
            if(c[l]>'9'){
                c[l]-=10;
                carry=1;
            } else {
                carry=0;
            }
            ++l;
        }

        if(j<0){
            ++k;
            while(--k>=0){
                c[l]=b[k]+carry;
                if(c[l]>'9'){
                    c[l]-=10;
                    carry=1;
                } else {
                    carry=0;
                }
                ++l;
            }
        }else{
            ++j;
            while(--j>=0){
                c[l]=a[j]+carry;
                if(c[l]>'9'){
                    c[l]-=10;
                    carry=1;
                } else {
                    carry=0;
                }
                ++l;
            }
        }
        if(carry==1) c[l++]='1';
        if(l>80) cout<<"overflow\n";
        else{
            while(--l>=0) cout<<c[l];
            cout<<'\n';
        }
    }
    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string>
#include <iostream>
#include <math.h>
#include <stack>

using namespace std;

int main(){
    
    char d;
    char e;
    using namespace std;
    stack<int> st1;
    stack<int> st2;
    stack<int> st3;
    int n;
    //scanf("%d",&n);
    while((d=getchar())!='\n'){
        n=atof(&d);
    }
    
    for(int i=0; i<n; i++){
        
        while((d=getchar())!='\n'){
            //printf("a%c\n",d);
            st1.push(atof(&d));
        }
        
        //while(getchar()!='\n'){}
        
        while((e=getchar())!='\n'){
            //printf("b%c\n",e);
            st2.push(atof(&e));
        }
        int temp=0;
        int num=0;
        
        while(st1.empty() != true || st2.empty() != true){
            if(st1.empty()==true && st2.empty()!=true){
                temp+=st2.top();
                st2.pop();
            }else if(st1.empty()!=true && st2.empty()==true){
                temp+=st1.top();
                st1.pop();
            }else{
                temp+=st1.top()+st2.top();
                st1.pop();
                st2.pop();
            }
            st3.push(temp%10);
            temp=temp/10;
            num++;
        }
        //if(temp!=0){
            st3.push(temp);
        //}
        if(num>80){
            printf("overflow");
        }else{
            while(st3.empty()!=true){
                cout << st3.top();
                st3.pop();
            }
        }
        printf("\n");
        
        
        
    }
    
}