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 <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void)
{
    char num1[81],num2[81];
        int a1[81],a2[81],ans[81];
        int i,j,n,k,l1,l2,kuriagari=0;
        scanf("%d",&n);
        for(i=0;i<n;i++)
        {
                for(j=0;j<81;j++)
                {
                        a1[j] = 0;
                        a2[j] = 0;
                        ans[j] = 0;
                }
                scanf("%s",num1);
                scanf("%s",num2);
                l1 = strlen(num1);
                l2 = strlen(num2);
                for(j=0;j<l1;j++)
                {
                        a1[j] = num1[l1-1-j] - '0';
                }
                for(j=0;j<l2;j++)
                {
                        a2[j] = num2[l2-1-j] - '0';
                }
                for(j=0;j<81;j++)
                {
                        ans[j] = (a1[j] + a2[j] + kuriagari) % 10;
                        kuriagari = (a1[j] + a2[j] + kuriagari)/10;
                }
                if(ans[80] != 0)
                {
                        printf("overflow\n");
                }
                else
                {
                        for(j=80;j>=0;j--)
                        {
                                if(ans[j] != 0)
                                {
                                        k = j;
                                        break;
                                }
                        }
                        for(j=k;j>=0;j--)
                        {
                                printf("%d",ans[j]);
                        }
                        printf("\n");
                }
        }
        return 0;
}

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

int main(){


  string strx,stry;
  int N;
  cin >> N;
  while(N){

    cin >> strx ;
    cin >> stry;
    int x[30] = {0},y[30] = {0};
    int i,lenx = strx.size(), leny = stry.size();
    string tmp;
    
    for( i = 0; lenx > 5 ; i++){
      tmp = strx.substr(lenx - 5);
      x[i] = atoi(tmp.c_str());
      strx =  strx.substr(0,lenx - 5);
      lenx -= 5;
    }
    x[i] = atoi(strx.c_str());
    for( i = 0; leny > 5 ; i++){
      tmp = stry.substr(leny - 5);
      y[i] = atoi(tmp.c_str());
      stry =  stry.substr(0,leny - 5);
      leny -= 5;
    }
    y[i] = atoi(stry.c_str());
    
    
    int ans[20] = {0};
    for(i = 0 ; i < 20 ; i++){
      ans[i] += x[i] + y[i];
      if(ans[i] > 99999){
	ans[i+1] += 1;
	ans[i] -= 100000;
      }
    }
    if(ans[16] != 0)
      cout << "overflow"<< endl;
    else{
      for(i = 15; i >= 0 ; i--){
	if(ans[i] != 0)
	  break;
      }
      cout << ans[i];
      for(i = i-1; i >= 0 ; i--){
	if(ans[i] < 10) cout << "0";
	if(ans[i] < 100) cout << "0";
	if(ans[i] < 1000) cout << "0";
	if(ans[i] < 10000) cout << "0";
	cout << ans[i];
      }
      cout << endl;
    }
    N--;
  }
  return 0;
}

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

int main()
{
	int n;
	cin >> n;

	for(int i=0;i<n;++i){
		string N1,N2;
		vector<int> num_line;
		cin >> N1 >> N2;

		int upper=0;
		int num;
		string longer  = N1.size() > N2.size() ? N1 : N2;
		string shorter = N1.size() < N2.size() ? N1 : N2;

		for(int i=0;i<longer.size();++i){
			if(i < shorter.size())
				num = longer[i]-'0' + shorter[i]-'0' + upper;
			else
				num = longer[i] - '0' + upper;
			
			if(num > 9){
				upper = 1;
				if(i != longer.size() -1)
					num -= 10;
			}else{
				upper = 0;
			}

			num_line.push_back(num);
		}

		if(num_line.size() > 80)
			cout << "overflow" << endl;
		else
			for(int i = num_line.size()-1;i>=0;--i)
				cout << num_line[i] ;

	}
	return 0;
}