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 <vector>
#include<string>
#include<sstream>
#define CAPACITY 9
 
 
namespace stoi{//stoi??????¨??????????????????????????????????????????§???£??¨???????
 
    int stoi(std::string str){
        int ret;
        std:: stringstream ss;
        ss << str;
        ss >> ret;
        return ret;
    }
 
}
class OverDigit{
 
public:
 
    int num_a[CAPACITY];
    int num_b[CAPACITY];
    OverDigit(std::string str_a, std::string str_b);
    void ChangeDigit(std::string str,int num[]);
    void Add();
};
 
OverDigit::OverDigit(std::string str_a,std::string str_b){
 
    ChangeDigit(str_a, num_a); 
    ChangeDigit(str_b, num_b);
 
}
 
void OverDigit::ChangeDigit(std::string str, int num[]){
 
    std::vector <int> num_str(CAPACITY, 0);
    int loc = CAPACITY, k = 8;
    int str_Max, str_rest;
 
    str_Max = str.length() / CAPACITY;//
    str_rest = str.length() % CAPACITY;
 
        for (int j = str.length() - CAPACITY; str_Max > 0; j = j-CAPACITY){
        num_str[k] = stoi::stoi(str.substr(j, CAPACITY));
        loc = k;
        k--;
        str_Max--;
    }
    //?????????????????????????????????????????\??????
     
        if(str_rest!=0)num_str[loc - 1] = stoi::stoi(str.substr(0, str_rest));
 
    for (int j = 0; j < CAPACITY; j++){
    num[j]=num_str[j];
    }
}
//80?????????§?????¶???????
void OverDigit::Add(){
 
    int num_ans[CAPACITY] = {};
    int fat,overflow=0,count=0;
 
    for (int j = CAPACITY-1; j >0; j--){
        if (num_a[j] + num_b[j]<=999999999){
            num_ans[j] = num_a[j] + num_b[j];
        }
        else if (num_a[j] + num_b[j] > 999999999){
            fat = (num_a[j] + num_b[j]) - 1000000000;
            num_ans[j] = fat;
            num_a[j - 1]++;     
        }
 
    }
 
     if (num_a[0] + num_b[0] > 99999999){
        overflow = 1;
     }
     else{
         num_ans[0]= num_a[0] + num_b[0];
          
 
     }
 
    if (overflow == 1){
        std::cout << "overflow" << std::endl;
    }
    else {
        for (int j = 0; j < CAPACITY; j++){
            if (num_ans[j] != 0){
                count++;
                std::cout << num_ans[j];
 
            }
            else if (num_ans[j] == 0 && count >= 1){
                std::cout << "000000000";
            }
 
        }std::cout << std::endl;
    }
 
}
 
 
int main(){
     
 
    //vector????????????¨??????80????????¨????
    int N;
    std::string str_a, str_b;
 
    std::cin >> N;
    std::cin.ignore();
 
    for (int i = 0; i < N; i++){
        getline(std::cin, str_a);
        getline(std::cin, str_b);
        OverDigit OverDigit_a(str_a,str_b);//????????????§??????
        OverDigit_a.Add();
    }
 
    return 0;
}

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

using namespace std;

#define REP(i,a,b) for((i)=(a);(i)<(int)(b);(i)++)
#define rep(i,n) REP(i,0,n)
// BEGIN CUT HERE
#define foreach(itr,c) for(typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
// END CUT HERE

int main(){
  int i, j, n;
  char tmp;

  scanf("%d",&n);
  rep(i,n){
    string str1, str2, res;
    cin >> str1;
    cin >> str2;

    vector<int> a(82, 0), b(82, 0);
    rep(j,str1.length()) a[j] = str1[j] - '0';
    rep(j,str2.length()) b[j] = str2[j] - '0';

    rep(j,81){
      int sum = a[j] + b[j];
      if(sum >= 10){ sum %= 10; a[j+1]++;}
      res += (sum + '0');
    }

    reverse(res.begin(), res.end());
    int f = 0;
    rep(j,res.length()){
      if(res[j] != '0') f = 1;
      if(f) cout << res[j];
    }

    cout << endl;
  }

  return 0;
}