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 <cstdlib>
using namespace std;

int main(){


  string strx,stry;
  int N;

  cin >> N;
  while(cin >> strx && cin >> stry){
 int x[20] = {0},y[20] = {0};
    int i=0,lenx = strx.size(), leny = stry.size();
    string tmp;
    
    for( i = 0; lenx > 5 ; i++){
      tmp = strx.substr(lenx - 5);
      x[i] = atoi(tmp.c_str());
      strx =  strx.substr(0,lenx - 5);
      lenx -= 5;
    }
    x[i] = atoi(strx.c_str());
    for( i = 0; leny > 5 ; i++){
      tmp = stry.substr(leny - 5);
      y[i] = atoi(tmp.c_str());
      stry =  stry.substr(0,leny - 5);
      leny -= 5;
    }
    y[i] = atoi(stry.c_str());
    
    
    int ans[20] = {0};
    for(i = 0 ; i < 20 ; i++){
      ans[i] += x[i] + y[i];
      if(ans[i] > 99999){
	ans[i+1] += 1;
	ans[i] -= 100000;
      }
    }
    if(ans[16] != 0)
      cout << "overflow"<< endl;
    else{
      for(i = 15; i >= 0 ; i--){
	if(ans[i] != 0)
	  break;
      }
      cout << ans[i];
      for(i = i-1; i >= 0 ; i--){
	if(ans[i] < 10) cout << "0";
	if(ans[i] < 100) cout << "0";
	if(ans[i] < 1000) cout << "0";
	if(ans[i] < 10000) cout << "0";
	cout << ans[i];
      }
    }
  }
  
  return 0;
}

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++) {
    if (i >= max_digit)
      throw runtime_error("overflow");
    rhs[i] = *p - '0';
  }
  return is;
}

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

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

  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[str1.length()-1-j] - '0';
      b[j] = str2[str2.length()-1-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());
    bool f = false;
    rep(j,res.length()){
      if(res[j] != '0') f = true;
      if(f) cout << res[j];
    }
    cout << endl;
  }

  return 0;
}