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 <iostream>
#include <iomanip>
#include <string.h>
#include <stdio.h>
using namespace std;

void checkArray(char str[])
// 配列の内容確認(テスト用)
{
	int size = strlen(str);
	for(int i = 0; i < size; ++i)
	{
		cout << "str[" << i << "]: " << str[i] << endl;
	}
}
void str_reverse(char s[])
// 文字列を反転する
{
     int i, l = strlen(s);
     char t;
     for(i = 0; i < l/2; i++) {
         t = s[i]; s[i] = s[l-i-1]; s[l-i-1] = t;
     }
}
void add(char num1[], char num2[])
// 足し算した結果を文字列として返す
{
	char res[82];;
	bool carry = false; //繰り上がりの有無
	int maxlen = (strlen(num1) > strlen(num2)) ? strlen(num1) : strlen(num2);
	int flag1 = 0, flag2 = 0;

	str_reverse(num1);
	str_reverse(num2);

	for(int i = 0; i < maxlen+1; ++i){
		// '\n'以降は値を0とする
		if(num1[i] == '\0') flag1 = 1;
		if(num2[i] == '\0') flag2 = 1;
		res[i] = carry;
		if(!flag1)	res[i] += num1[i]-48;
		if(!flag2)	res[i] += num2[i]-48;
		carry = res[i] / 10;
		res[i] = res[i] % 10 + 48;
	}
	// 終端文字をつける
	if(res[maxlen] == '0')
		res[maxlen] = '\0';
	else res[maxlen+1] = '\0';

	// 元に戻す
	str_reverse(num1);
	str_reverse(num2);
	str_reverse(res);
	if(strlen(res) > 80)
		cout << "overflow" << endl;
	else
		cout << res << endl;
}

int main() {
	int dataset;
	char num1[81];
	char num2[81];

	cin >> dataset; // データセットの個数

	for(int i = 0; i < dataset; ++i){
			// 80桁以下の数字を入力
			cin  >> num1;
			cin  >> num2;
			if(strlen(num1) > 80 || strlen(num2) > 80){
				cout << "overflow" << endl;
			}else
			// 足し算した結果を表示
			add(num1, num2);
	}
	return 0;
}

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

int main(){
	string a,b;
	vector<int> sum;
	int al, bl, sl;
	int n, up, num;
	cin >> n;
	for(int i=0;i<n;++i){
		cin >> a >> b;
		al = a.size()-1;
		bl = b.size()-1;
		up = 0;
		while(al >= 0 || bl >= 0){
			num = up;
			if(al>=0) num += a[al] - 48;
			if(bl>=0) num += b[bl] - 48;
			if(num > 9){
				sum.push_back(num - 10);
				up = 1;
			}else{
				sum.push_back(num);
				up = 0;
			}
			al -= 1;
			bl -= 1;
		}
		if(up == 1){
			sum.push_back(1);
		}
		sl = sum.size();
		if(sl > 80){
			cout << "overflow" << endl;
		}else{
			for(int j=sl-1;j>=0;--j){
				cout << sum[j];
			}
			cout << endl;
		}
		sum.clear();
	}
	return 0;
}

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

int main(){

	int n;

	string a, b;
	char N1[80], N2[80];
	char sum[80];

	cin >> n;

	for( int i=0;i<n;i++ ){

		cin >> a;
		cin >> b;

		if( a.length() >= 80 || b.length() >= 80 )
			cout << "overflow" << endl;

		for( int i=0;i<80;i++ ){
			N1[i] = -1; N2[i] = -1; sum[i] = 0;
		}

		for( int i=1;i<=a.length();i++ )
			N1[i] = a[a.length()-i] - '0';
		for( int i=1;i<=b.length();i++)
			N2[i] = b[b.length()-i] - '0';

		int over = 0;
		
		for( int i=0;i<80;i++ ){


			if( N1[i] == -1 && N2[i] == -1 ){
				if( over >= 10 ){
					sum[i] = over / 10;
					over = 0;
				}else
					sum[i] = -1;
				continue;
			}

			if( N1[i] != -1 )
				sum[i] += N1[i];
			if( N2[i] != -1 )
				sum[i] += N2[i];

			sum[i] += over / 10;

			over = floor((double)sum[i]/10)*10;

			sum[i] -= over;
			
		}

		if(over > 0)cout << "overflow" << endl;
		else{		
			for( int i=1;i<=80;i++ ){
				if( sum[80-i] >= 0 )
					cout << (int)sum[80-i];
			}
			cout << endl;
		}


	}
	return 0;
}