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 <stdlib.h>
#include <string>
#include <iostream>
#include <math.h>
#include <stack>

using namespace std;

int main(){
    
    string d;
    string e;
    
    stack<char> st1;
    stack<char> st2;
    stack<int> st3;
    int n=0;
    scanf("%d",&n);
    
    for(int i=0; i<n; i++){
        cin>>d;
        if((int)(d.size())>80){
            printf("overflow");
        }else{
            cin>>e;
            if((int)(e.size())>80){
                printf("overflow");
            }
        }
        
        if((int)(d.size())>80||(int)(e.size())>80){
            //printf("overflow");
        }else{
            for(int j=0; j<(int)(d.size()); j++){
                st1.push(d[j]);
            }
            for(int j=0; j<(int)(e.size()); j++){
                st2.push(e[j]);
            }
            int temp=0;
            char temp_char;
            while(st1.empty()==false||st2.empty()==false){
                if(st1.empty()==false&&st2.empty()==false){
                    temp_char=st1.top();
                    temp+=atoi(&temp_char);
                    temp_char=st2.top();
                    temp+=atoi(&temp_char);
                    st1.pop();
                    st2.pop();
                }else if(st1.empty()==false&&st2.empty()==true){
                    temp_char=st1.top();
                    temp+=atoi(&temp_char);
                    st1.pop();
                }else if(st1.empty()==true&&st2.empty()==false){
                    temp_char=st2.top();
                    temp+=atoi(&temp_char);
                    st2.pop();
                }
                st3.push(temp%10);
                temp=temp/10;
            }
            
            if(temp!=0){
                st3.push(temp);
            }
            
            if(st3.size()>80){
                printf("overflow");
            }else{
                while(st3.empty()==false){
                    printf("%d",st3.top());
                    st3.pop();
                }
            }
            
        }
        printf("\n");
    }
    
}

C++: #include <bits/stdc++.h>

//const static double	de_PI	= 3.14159265358979323846;
//const static int	de_MOD	= 1000000007;
//const static int	de_MAX	= 999999999;
//const static int	de_MIN = -999999999;

inline std::string Add_MultiplePrecision(const std::string A, const std::string B) {

	int a[200] = {};
	int b[200] = {};
	int c[300] = {};

	for (unsigned int i = 0; i < A.length(); i++) {
		a[i] = A[A.length() - 1 - i] - '0';
	}
	for (unsigned int i = 0; i < B.length(); i++) {
		b[i] = B[B.length() - 1 - i] - '0';
	}

	int carry = 0;
	for (unsigned int i = 0; i < std::max(A.length(), B.length()); i++) {
		c[i] = (a[i] + b[i] + carry) % 10;
		c[i + 1] = carry = (a[i] + b[i] + carry) / 10;
	}

	int st = 299;
	while (c[st] == 0) {
		st--;
	}

	std::string ans;
	for (int i = st; i >= 0; i--) {
		ans += std::to_string(c[i]);
	}

	if (ans == "") {
		ans = "0";
	}

	return ans;
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int n = 0;
	std::cin >> n;
	std::vector<std::string> ans(n);

	for (int i = 0; i < n; i++) {
		std::string temp1, temp2, temp3;
		std::cin >> temp1 >> temp2;
		temp3 = Add_MultiplePrecision(temp1, temp2);
		if (temp3.length() >= 80) {
			ans[i] = "overflow";
		}
		else {
			ans[i] = temp3;
		}
	}

	for (int i = 0; i < n; i++) {
		std::cout << ans[i] << std::endl;
	}
}

C++: //0015 National Budget
//80ÜÅÌ2ÂÌ®Ì«µZÊðoÍ¹æ
//1sÚÉf[^Zbgnª^¦çêé

#include<iostream>
#include<string>
#include<algorithm>
using namespace std;

int main(void)
{
	int n;
	cin >> n;
	while(n--)
	{
		const int N = 82;
		string str;
		int arg[2][N] = {0}, result[N] = {0};
		int digit[2];

		for(int i = 0; i < 2; i++)
		{
			cin >> str;
			digit[i] = str.size();
			for(int j = 0; j < digit[i]; j++)
			{
				arg[i][j] = str[j] - '0';
			}
			//vZµâ·¢æ¤É½]
			reverse(arg[i], arg[i]+digit[i]);
		}

		bool isRead[2] = {true, true};
		for(int i = 0; isRead[0] || isRead[1]; i++)
		{
			if(i > digit[0])
				isRead[0] = false;
			if(i > digit[1])
				isRead[1] = false;

			int s = 0;
			if(isRead[0])
				s += arg[0][i];
			if(isRead[1])
				s += arg[1][i];

			result[i] += s%10;
			result[i+1] += s/10 + result[i]/10;
			result[i] %= 10;
		}

		bool isPrint = false;
		for(int i = N-1; i >= 0; i--)
		{
			if(result[i])
				isPrint = true;
			if(isPrint)
				cout << result[i];
		}
		cout << endl;
	}
	return 0;
}