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 <cstdlib>
#include <cstdint>
#include <cstring>
#include <cctype>
#include <iostream>
#include <sstream>
#include <vector>
#include <list>
#include <deque>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <functional>
#include <stdexcept>
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long long ll;
const double PI = 3.141592653589793238462643383279502884L;
const int LARGE_INT = 1000000007;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define tr(c, i) for(auto i = (c).begin(); i != (c).end(); i++)
#define rtr(c, i) for(auto i = (c).rbegin(); i != (c).rend(); i++)
#define repfn(i,a,cont,next) \
  for(auto i = (a); [=](){return(cont);}(); i = [=](){return(next);}())
#define repby(i,a,b,s) repfn(i,a,i<b,i+s)
#define repab(i,a,b) repby(i,a,b,1)
#define rep(i,b) repab(i,0,b)
#define pb push_back
#define sz(c) int((c).size())

typedef vector<int> bignum;
const int max_digit = 80;

bignum operator+(const bignum& lhs, const bignum& rhs)
{
  int tmp, carry = 0;
  bignum res(max_digit, 0);
  for (int i = 0; i < max_digit; i++) {
    tmp = lhs[i] + rhs[i] + carry;
    if (tmp > 9) {
      carry = 1;
      res[i] = tmp % 10;
    } else {
      carry = 0;
      res[i] = tmp;
    }
  }
  if (carry)
    throw runtime_error("overflow");
  return res;
}

ostream& operator<<(ostream& os, const bignum& rhs)
{
  bool skip = true;
  rtr(rhs, i) {
    if (*i == 0 && skip)
      continue;

    skip = false;
    os << *i;
  }
  return os;
}

istream& operator>>(istream& is, bignum& rhs)
{
  string s;
  int i = 0;
  is >> s;
  rhs.assign(max_digit, 0);
  for (auto p = s.rbegin(); p != s.rend(); p++, i++) {
    rhs[i] = *p - '0';
  }
  if (sz(rhs) > max_digit)
    throw runtime_error("overflow");
  return is;
}

int main(int argc, char **argv)
{
  int n;
  bignum a, b;

  cin >> n;
  try {
    while (n--) {
      cin >> a >> b;
      cout << (a + b) << endl;
    }
  } catch (runtime_error& re) {
    cout << re.what() << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<stack>
#include<algorithm>
using namespace std;
string str1;
string str2;
stack<int> sta;

int main()
{
  int n;
  int i,j;
  int exc;//??°?????????
  int sum;

  cin>>n;
  for(i=0;i<n;i++){
    cin>>str1;
    cin>>str2;
    if(str1.size()>80 || str2.size()>80){
      cout<<"overflow"<<endl;
      continue;
    }
    exc=0;
    for(j=1;j<=min(str1.size(),str2.size());j++){
      sum=str1[str1.size()-j]-'0'+str2[str2.size()-j]-'0'+exc;
      exc=sum/10;
      sta.push(sum%10);
    }
    if( str1.size() < str2.size() ){
      while(j<=str2.size()){
	sum=str2[str2.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }else if( str1.size() > str2.size() ){
      while(j<=str1.size()){
	sum=str1[str1.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }
    if(max(str1.size(),str2.size())==80 && exc==1){
      cout<<"overflow"<<endl;
      continue;
    }
    if(exc)
      cout<<exc;
    for(j=0;j<max(str1.size(),str2.size());j++){  
      cout<<sta.top();
      sta.pop();
    }
    cout<<endl;
  }
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
using namespace std;

int main(void){
	int N;
	cin >> N;
	for(int n=0;n<N;n++){
		string s1,s2,s3;
		cin >> s1 >> s2;
		reverse(s1.begin(),s1.end());
		reverse(s2.begin(),s2.end());
		bool up=false;
		for(int i=0;i<s1.size()||i<s2.size();i++){
			if(i<s1.size()&&i<s2.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s1[i]+s2[i]-'0'-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}else if(i<s1.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s1[i]-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}else if(i<s2.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s2[i]-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}
		}
		if(up)
			s3+='1';
		if(s3.size()>80)
			cout << "overflow" << endl;
		else{
			reverse(s3.begin(),s3.end());
			cout << s3 << endl;
		}
	}
	return 0;
}