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 <bits/stdc++.h>
using namespace std;

#define FOR(i,l,r) for(int i = (l);i < (r);i++)
#define ALL(x) (x).begin(),(x).end()
template<typename T> bool chmax(T& a,const T& b){return a < b ? (a = b,true) : false;}
template<typename T> bool chmin(T& a,const T& b){return b < a ? (a = b,true) : false;}
typedef long long ll;

int N;
const int MAX = 100;
string S,T;

deque<int> sum_str(string s,string t)
{
	deque<int> res;
	bool carry = false;
	for(int i = 0;i < max(s.size(),t.size()) || carry;i++){
		res.push_front(carry);
		carry = false;
		if(i < s.size()){
			res.front() += s [s.size() - i - 1] - '0';
		}
		if(i < t.size()){
			res.front() += t [t.size() - i - 1] - '0';
		}
		if(res.front() >= 10){
			res.front() %= 10;
			carry = true;
		}
	}
	
	return res;
}

int main()
{
	cin >> N;
	FOR(i,0,N){
		cin >> S >> T;
		deque<int> ans = sum_str(S,T);
		if(ans.size() <= 80){
			FOR(i,0,ans.size()){
				cout << ans [i] << (i == ans.size() - 1 ? "\n" : "");
			}
		}
		else{
			cout << "overflow" << endl;
		}
	}

	return 0;
}

C++: /*
	0015:National Budget
*/
#include	<iostream>
#include	<string>
#include	<iterator>

using namespace std;

int main(void) {
	int n;
	
	cin >> n;
	for(int i = 0; i < n; i++) {
		string num1, num2, result;
		bool isMoveUp = false;
		
		cin >> num1 >> num2;
		string::reverse_iterator st1 = num1.rbegin();
		string::reverse_iterator st2 = num2.rbegin();
		while( st1 != num1.rend() || st2 != num2.rend()) {
			char buf1 = st1 != num1.rend() ? (*st1) : '0';
			char buf2 = st2 != num2.rend() ? (*st2) : '0';
			int buf3 = (buf1 - '0') + (buf2 - '0');
			
			isMoveUp = false;
			if(buf3 >= 10) {
				isMoveUp = true;
				(*(st1+1))++; buf3 -= 10;
			}
			result += (buf3 + '0');
			if(st1 != num1.rend()) { st1++; }
			if(st2 != num2.rend()) { st2++; }
		}
		if(isMoveUp) { result += '1'; }
		if(result.size() > 80) { cout << "overflou" << endl; continue; }
		string::reverse_iterator rt = result.rbegin();
		while(rt != result.rend()) {
			cout << (*rt);
			rt++;
		}
		cout << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
#define N 80

int main(void){
    char buf[N*2];
    int datas;
    int degits;
    int lint1[N+1], lint2[N+1], sum[N+1];
    int cry;
    
    scanf("%d", &datas);
    for (int n=0; n<datas; n++){
        // reset
        for (int i=0; i<N; i++) lint1[i]=lint2[i]=0;
        // input
        scanf("%s", buf);
        degits = strlen(buf);
        if (degits > 80){
         //   scanf("%s", buf); // read another num.
            puts("overflow");
            continue;
        }
        for (int i=0; i<degits; i++){
            lint1[degits-i-1]= buf[i]-'0';
        }
        scanf("%s", buf);
        degits = strlen(buf);
        if (degits > 80){
            puts("overflow");
            continue;
        }
        for (int i=0; i<degits; i++){
            lint2[degits-i-1]= buf[i]-'0';
        }
        // calc
        cry=0;
        for (int i=0; i<N; i++){
            sum[i] = lint1[i]+lint2[i] + cry;
            cry = sum[i]>9 ? 1:0;
            if (cry) sum[i] -= 10;
        }
        // output
        if (cry){
            puts("overflow");
        } else {
            bool zero=true;
            if (cry) puts("1");
            for (int i=N-1; i>=0; i--){
                if (zero && sum[i] != 0) zero=false;
                if (!zero) printf("%d", sum[i], i);
            }
            printf("\n");
        }
        
    }
    
    return 0;
}

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

int main(){
  int j;
  int n;
  char s1[111];
  char tmp[111];
  char s2[111];
  char ans[111];
  int  kuri = 0;
  cin >> n;
  while(n--){
  memset(ans,0,sizeof(ans));
  memset(s1,0,sizeof(s1));
  memset(s2,0,sizeof(s2));
  memset(tmp,0,sizeof(tmp));)
  cin >> s1 >> s2;
  if(strlen(s1) < strlen(s2)){
      for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
      for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
      for(int i=0;i<strlen(s2);i++){
	if((s2[i] + (s1[i]%'0') + kuri) > '9'){
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri)-10;
	  kuri = 1;
	}
	else {
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri);
	  kuri = 0;
	}
	if(i == strlen(s2)-1 && kuri == 1) ans[i+1] = '1';
      }
      if(strlen(ans) > 80) cout << "overflow" << endl;
      else{
	for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
      }
  }
  else{
    for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
    for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
    for(int i=0;i<strlen(s1);i++){
      if((s1[i] + (s2[i]%'0') + kuri) > '9'){
	ans[i] = (s1[i] + (s2[i]%'0') + kuri)-10;
	kuri = 1;
      }
      else {
	ans[i] = (s1[i] + (s2[i]%'0') + kuri);
	kuri = 0;
      }
      if(i == strlen(s1)-1 && kuri == 1) ans[i+1] = '1';
    }
    if(strlen(ans) > 80) cout << "overflow" << endl;
    else{
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
    }
  }
  cout << endl;
  }
}