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 <string>
#include <stack>
using namespace std;

int eval(char a)
{
    return a - '0';
}

int main(void)
{
    string a, b;
    int n;
    cin >> n;
    for (int i = 0; i < n; ++i) {
        stack<int> c;
        cin >> a >> b;
        auto ia = a.rbegin();
        auto ib = b.rbegin();
        int g = 0;
        while (true) {
            int e = 0, f = 0;
            if (ia != a.rend())
                e = eval((char)*ia++);
            if (ib != b.rend())
                f = eval((char)*ib++);
            c.push((e + f + g)%10);
            g = (e + f + g >=10)? 1 : 0;
            if (ia == a.rend() && ib == b.rend())
            {
                if (g == 1)
                    c.push(1);
                break;
            }
        }
        int l = c.size();
        if (l > 80) {
            cout << "overflow" << endl;
            continue;
        }
        for (int i = 0; i < l; ++i) {
            cout << c.top();
            c.pop();
        }
        cout << endl;
    }
    return 0;
}

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;
        cin>>e;
        
        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 <iostream>
#include <string>
#include <sstream>
#include <deque>
class BigInt
{
  std::deque<char> m_digits;
public:
  BigInt(std::string str) {
    for(auto it = str.begin(); it != str.end(); ++it) {
      m_digits.push_back(*it - '0');
    }
  }

  BigInt(int num) {
    std::ostringstream oss;
    oss << num;
    BigInt(oss.str());
  }

  std::string str() const {
    std::string str;
    for (auto it = m_digits.begin(); it != m_digits.end(); ++it) {
      str += *it + '0';
    }
    return str;
  }

  const BigInt& operator +=(const BigInt& rhs) {
    std::deque<char> new_digits;
    auto lhs_rit = m_digits.rbegin();
    auto rhs_rit = rhs.m_digits.rbegin();

    char inc = 0;
    while (lhs_rit != m_digits.rend() &&
           rhs_rit != rhs.m_digits.rend()) {
      char temp = *lhs_rit + *rhs_rit;
      temp += inc;
      if (temp >= 10) {
        inc = temp / 10;
        temp -= inc * 10;
      }
      new_digits.push_front(temp);
      ++lhs_rit;
      ++rhs_rit;
    }

    if (lhs_rit != m_digits.rend()) {
      while (lhs_rit != m_digits.rend()) {
        char temp = *lhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++lhs_rit;
      }
    }
    else if (rhs_rit != rhs.m_digits.rend()) {
      while (rhs_rit != rhs.m_digits.rend()) {
        char temp = *rhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++rhs_rit;
      }
    }
    if (inc > 0) new_digits.push_front(inc);
    m_digits = new_digits;
  }
};

BigInt operator +(const BigInt& lhs, const BigInt& rhs)
{
  BigInt result(lhs);
  result += rhs;
  return result;
}

int main()
{
  int n;
  std::cin >> n;
  while (n--) {
    std::string num;
    std::cin >> num;
    BigInt num1(num);
    std::cin >> num;
    BigInt num2(num);
    
    BigInt num3 = num1 + num2;
    std::cout << num3.str() << std::endl;
  }
  return 0;
}