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 <cstdio>
#include <cstring>
using namespace std;

int main(void)
{
    int n;
    int flag;
    scanf("%d", &n);
    
    while (n != 0){
        flag = 1;
        
        int num1[81] = {0};
        int num2[81] = {0};
        int max_len;
        
        char in[3000];
        scanf("%s", in);
        // êUlðæèÞ
        int len = (int)strlen(in);
        if (80 < len) flag = 0;
        // üÁ½lª80Èã¾Á½çAflagð0É·é
        else {
            max_len = len;
            int j = 0;
            for (int i = (len - 1); -1 < i; i--){
                num1[j] = in[i] - '0';
                j++;
            }
        }
        
        scanf("%s", in);
        // êUlðæèÞ
        len = (int)strlen(in);
        if (80 < len) flag = 0;
        // üÁ½lª80Èã¾Á½çAflagð0É·é
        else {
            if (max_len < len) max_len = len;
            int j = 0;
            for (int i = (len - 1); -1 < i; i--){
                num2[j] = in[i] - '0';
                j++;
            }
        }
        
        // üÁ½lª80Èã¶áÈ¢Æ«ÉZ
        if (flag == 1){
            for (int i = 0; i < 80; i++){
                num1[i] += num2[i];
                if (10 <= num1[i]){
                    num1[i + 1] += 1;
                    num1[i] %= 10;
                }
            }
            
            if (num1[80] != 0){
                flag = 0;
                // 81Úª0¶áÈ¢ = overflow
            }
            
            // overflowµÄÈ¢Æ«ÉoÍ
            if (flag == 1){
                // ÅãÌ«µã°ÅJèãªèµÄé©Ç¤©
                if (num1[max_len] == 0){
                    for (int i = max_len - 1; -1 < i; i--){
                        printf("%d", num1[i]);
                    }
                    printf("\n");
                }
                else {
                    for (int i = max_len; -1 < i; i--){
                        printf("%d", num1[i]);
                    }
                    printf("\n");
                }
            }
        }
        
        // ¡ÜÅÉflagª0ÉÈÁÄ½çoverflowI
        if (flag == 0){
            printf("overflow\n");
        }
        
        n--;
    }
    
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){

  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string a,b,out,tmp;
    cin >> a;
    cin >> b;
    //int m=max(a.size(),b.size());
    if(a.size()<b.size()){
      tmp=a;
      a=b;
      b=tmp;
    }
    //cout << a << endl;
    int si=-a.size()+b.size();
    for(int j=a.size()-1;j>=0;j--){
      if(j+si>=0){
	//cout << si << endl;
	a[j]+=b[j+si]-'0';
	//cout << a[j] << endl;
      }else{
	a[j]+=0;
	//cout << "!" << endl;
      }
      if(a[j]>'9'&&j!=0){
	a[j-1]++;
	a[j]-=10;
      }else if(a[j]>'9'&&j==0){
	a='1'+a;
	a[1]-=10;
	//for(int i=a.size();i>0;i--)
	//  a[i]=a[i-1];
	//a[1]-=10;
	//a[0]='1';
      }
    }
    if(a[80]!='\0')
      cout << "overflow" << endl;
    else
      cout << a << endl;
  }

}

C++: #include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <queue>
#include <map>
#include <fstream>
#include <array>
#include <utility>
#include <functional>
#include <stack>

using namespace std;
typedef long long Int;
#define REP(i , x) for(Int i = 0 ; i < x ; ++i)

typedef pair<int, int> P;


int main()
{
	int n;
	cin >> n;
	REP(i, n){
		string a, b;
		int a_a[100] = { 0 };
		int b_a[100] = { 0 };
		int c_a[100] = { 0 };
			cin >> a >> b;
			if (a.length() >= 80 || b.length() >= 80){
				cout << "overflow" << endl;
				continue;
			}

			REP(i, a.length()){
				a_a[i] = a[a.length() - i -1] - '0';
			}
			
			REP(i, b.length()){
				b_a[i] = b[b.length() - i -1] - '0';
			}

			REP(i, 84){
				int temp = a_a[i] + b_a[i] + c_a[i];
				c_a[i + 1] += temp / 10;
				c_a[i] = temp % 10;
			}

			int i = 84;

			while (!c_a[i] && i){
				--i;
			}
			if (i >= 80){
				cout << "overflow" << endl;
				continue;
			}

			while (i >= 0){
				cout << c_a[i];
				--i;
			}
			cout << endl;
		}

	return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    int ans[200] = {0};
    string a,b;
    cin >>a>>b;
    int as = a.size(),bs = b.size(),t = 0,s = 0,x = 0;
    int ms = (as>bs)? as:bs;
    for(int i=0; i<ms; i++,x++){
      if(i<as && i<bs){t = a[as-i-1]+b[bs-i-1]-2*'0'+s;}
      else if(i>=as){t = b[bs-i-1]-'0'+s;}
      else if(i>=bs){t = a[as-i-1]-'0'+s;}
      s = t/10;
      ans[x] = t%10;
    }
    if(s){ans[x] = s;x++;}
    if(x>=80){cout <<"over flow"<<endl;}
    else{
      for(int i=0; i<x; i++){cout <<ans[x-i-1];}
      cout <<endl;
    }
  }
  return 0;
}