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>

int main(void)
{
  int N;
  scanf("%d", &N);

  int i,j,k;
  for(i=0; i<N; ++i){

    int digits[2][80];
    for(j=0; j<2; ++j){
      for(k=0; k<80; ++k){
        digits[j][k] = 0;
      }
    }

    int overflow = 0; 

    for(j=0; j<2; ++j){
      char digit[81];
      scanf("%s%*[^\n]", digit);
      getchar();
      
      if(overflow || strlen(digit)>80){
        overflow = 1;
      }else{
        for(k=0; k<strlen(digit); ++k){
          digits[j][k] = digit[strlen(digit)-1-k]-'0';
        }
      }
    }

    if(!overflow){
      int sum[81];
      for(j=0; j<81; ++j){
        sum[j] = 0;
      }

      for(j=0; j<80; ++j){
        for(k=0; k<2; ++k){
          sum[j] += digits[k][j];
        }
        sum[j+1] += sum[j]/10;
        sum[j] %= 10;
      }

      if(sum[80]>0){
        overflow = 1;
      }else{
        int available = 0;
        for(j=80; j>=0; --j){
          if(available || sum[j]!=0 || j==0){
            available = 1;
            printf("%d", sum[j]);
          }
        }
        printf("\n");
      }
    }

    if(overflow){
      printf("overflow\n");
    }
  }

  return 0;
}

C++: #include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

using namespace std;

class LargeInteger
{
private:
	vector<int> places;
public:
	LargeInteger( string number )
	{
		reverse( number.begin(), number.end() );

		for ( unsigned int i = 0; i < number.length(); i++ )
		{
			this -> places.push_back( number[i] - '0' );
		}

		return;
	}

	LargeInteger& operator += ( string number )
	{
		reverse( number.begin(), number.end() );

		for ( unsigned int i = 0; i < number.length(); i++ )
		{
			if ( i < places.size() )
			{
				this -> places[i] += number[i] - '0';
			}
			else
			{
				this -> places.push_back( number[i] - '0' );
			}
		}

		return *this;
	}

	void printNumber()
	{
		adjust();

		if ( places.size() < 80 )
		{
			for ( vector<int>::reverse_iterator it = places.rbegin(); it != places.rend(); it++ )
			{
				cout << *it;
			}
		}
		else
		{
			cout << "overflow";
		}
		cout << endl;
		
		return;
	}
	
	void adjust()
	{
		for ( unsigned int i = 0; i < places.size(); i++ )
		{
			if ( 10 <= places[i] )
			{
				if ( i + 1 < places.size() )
				{
					places[i+1] += places[i] / 10;
				}
				else
				{
					places.push_back( places[i] / 10 );
				}
				places[i] %= 10;
			}
		}

		return;
	}
};


int main()
{
	int n;
	string buffer;

	cin >> n;

	for ( int i = 0; i < n; i++ )
	{
		cin >> buffer;
		LargeInteger li( buffer );

		cin >> buffer;
		li += ( buffer );

		li.printNumber();
	}

	return 0;
}