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 <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<string, ll> PS;
typedef vector<ll> V;

int k = 0, n;
string s, t;
vector<string> ans;

int main(){
	cin >> n;
	REP(i,n){
		cin >> s >> t;
		k = 0;
		if (s.size() >= 80 || t.size() >= 80){
			cout << "overflow" << endl;
		}
		reverse(ALL(s));
		reverse(ALL(t));
		int a = max(s.size(), t.size()), b = min(s.size(), t.size());
		if (s.size() > t.size()){
			REP(i, a - b){
				t += "0";
			}
		}
		else{
			REP(i, a - b){
				s += "0";
			}
		}		
		reverse(ALL(s));
		reverse(ALL(t));
		//cout << s << endl;
		//cout << t << endl;
		REP(i, s.size()){
			int x = (s[s.size()-1-i] - '0') + (t[s.size()-1-i] - '0');
			string q;
			stringstream ss;
			if (i != s.size() - 1){
				ss << ((x + k) % 10);
			}
			else{
				ss << x + k;
			}
			ss >> q;
			ans.push_back(q);
			if (x + k >= 10){
				k = (x + k) / 10;
			}
			else{
				k = 0;
			}
		}
		reverse(ALL(ans));
		REP(i, ans.size()){
			cout << ans[i];
		}
		cout << endl;
		ans.clear();
	}
}

C++: #include<iostream>
#include<string>
#include<cmath>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
using namespace std;

int stoi(string s){
  int size=s.size();
  int num=0;
  rep(i,size){
    num+=((int)s[i]-'0')*pow(10,size-i-1);
  }
  return num;
}

string itos(int num){//this function is used when you want to change variable type from int to string
  int size=0;
  int cnt=1;
  while(num/cnt!=0){//count the digit of num
    size++;
    cnt*=10;
  }
  string s="";
  rep(i,size){//change num from int to string
    s+=(char)((num/(int)pow(10,size-i-1))+'0');
    num-=(int)(num/pow(10,size-i-1))*pow(10,size-i-1);
  }
  return s;
}

int main(){
	int n;
	cin>>n;
	string s[2];
	rep(i,n){
		cin>>s[0]>>s[1];
		if(s[0].size()>80 || s[1].size()>80){
			cout<<"overflow"<<endl;
			continue;
		}
		int dis=abs(s[0].size()-s[1].size());
		if(s[0].size()>s[1].size()){
			rep(j,dis){
				s[1].insert(0,"0");
			}
		}else{
			rep(j,dis){
				s[0].insert(0,"0");
			}
		}
		int size=s[0].size();
		int alt=0;
		string ans="";
		int num1,num2;
		for(int a=(size-1);a>=0;a--){
			num1=(int)s[0][a]-'0';
			num2=(int)s[1][a]-'0';
			int result=num1+num2+alt;
			if(result<10){
				if(!result)ans.insert(0,"0");
				else ans.insert(0,itos(result));
				alt=0;
			}else{
				if(!(result%10))ans.insert(0,"0");
				else ans.insert(0,itos(result%10));
				alt=1;
			}
		}
		if(alt)ans.insert(0,"1");
		if(ans.size()>80)cout<<"overflow"<<endl;
		else cout<<ans<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <list>
using namespace std;

class bigdata
{
public:
  list < int >data;
  string to_str ()
  {
    string str (data.size (), '0');
    int i = 0;
    auto it = data.begin ();
    while (it != data.end ())
      {
	string buf = to_string (*it);
	  str[i] = buf[0];
	  it++;
	  i++;
      }
    return str;
  }
  bigdata & operator= (string str)
  {
    for (int i = 0; i < str.length (); i++)
      data.push_back (stoi (str.substr (i, 1)));
  }
  bigdata & operator+ (bigdata & obj)
  {
    auto it_a = this->data.begin ();
    auto it_b = obj.data.begin ();
    if (this->data.size () < obj.data.size ())
      {
	int buf = obj.data.size () - this->data.size ();
	for (int i = 1; i <= buf; i++)
	  this->data.push_front (0);
      }
    if (this->data.size () > obj.data.size ())
      {
	int buf = this->data.size () - obj.data.size ();
	for (int i = 1; i <= buf; i++)
	  obj.data.push_front (0);
      }
    this->data.push_front (0);
    obj.data.push_front (0);

    while (it_a != this->data.end ())
      {
	*it_a += *it_b;
	if (*it_a >= 10)
	  {
	    --it_a;
	    *it_a += 1;
	    ++it_a;
	    *it_a -= 10;
	  }
	it_a++;
	it_b++;
      }
    it_a = this->data.begin ();
    if (*it_a == 0)
      this->data.pop_front ();
    return *this;
  }
};

int
main ()
{
  bigdata data1, data2;
  string s1, s2;
  cin >> s1 >> s2;
  data1 = s1;
  data2 = s2;
  data1 = data1 + data2;
  cout << data1.to_str () << endl;
}