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<cstring>
#include<cstdlib>

int main(void)
{
	char num1[81], num2[81];
	int n, num1_k, num2_k, rec, nnum1, nnum2, k_up, bnum1, bnum2;
	int f1, f2;
	scanf("%d", &n);
	fflush(stdin);
	for(f1 = 0; f1 < n; f1++)
	{
		//初期化
		k_up = 0;
		//入力
		scanf("%s", num1);
		scanf("%s", num2);
		//それぞれの桁数を求める
		num1_k = strlen(num1);
		num2_k = strlen(num2);
		bnum1 = num1_k - 1;
		bnum2 = num2_k - 1;
		//桁数で低いほうの桁数分だけ回す
		rec = (num1_k > num2_k) ? num2_k : num1_k;
		for(f2 = rec - 1; f2 >= 0; f2--)
		{
			//それぞれ現在桁の数を割り出す
			nnum1 = num1[bnum1] - '0';
			nnum2 = num2[bnum2] - '0';
			//桁数の大きいほうへ入れる
			if(num1_k > num2_k)
			{
				num1[bnum1] = ((nnum1 + nnum2) % 10 + k_up) + '0';
			}
			else
			{
				num2[bnum2] = ((nnum1 + nnum2) % 10 + k_up) + '0';
			}
			//桁上がりを求める
			k_up = (nnum1 + nnum2) / 10;
			bnum1--;
			bnum2--;
		}
		//残りの部分を求める
		rec = (bnum1 > bnum2) ? bnum1 : bnum2;
		for(f2 = rec; f2 >= 0; f2--)
		{	
			//桁数の大きいほうへ入れる
			if(num1_k > num2_k)
			{
				nnum1 = num1[f2] - '0';
				num1[f2] = ((k_up + nnum1) % 10) + '0';
				k_up = (k_up + nnum1) / 10;
			}
			else
			{
				nnum2 = num2[f2] - '0';
				num2[f2] = ((k_up + nnum2) % 10) + '0';
				k_up = (k_up + nnum2) / 10;
			}			
		}
		if(k_up == 1)
		{
			if(num1_k + 1 > 80 || num2_k + 1 > 80)
			{
				printf("overflow\n");
			}
			else
			{
				printf("%d", k_up);
			}
		}
		if(num1_k > num2_k)
		{
			printf("%s\n", num1);
		}
		else
		{
			printf("%s\n", num2);
		}
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<sstream>
#include<iomanip>
using namespace std;
class BigInt{
  unsigned int i[11];
public:
  BigInt(string str){
    i[0]=0;
    for(int j=80;j>0;j-=8){
      if((int)str.size()>j){
	stringstream ss;
	ss << str.substr(str.size()-j,8);
	ss >> i[11-j/8];
      }else if((int)str.size()+8>j){
	stringstream ss;
	ss << str.substr(0,str.size()+8-j);
	ss >> i[11-j/8];
      }else{
	i[11-j/8]=0;
      }
    }
  }
  BigInt():i{0,0,0,0,0,0,0,0,0,0,0}
  {}
  void print(){
    if(i[0]>0)
      cout << "overflow\n";
    else{
      int j=1;
      for(;j<11;j++)
	if(i[j]>0)
	  break;
      cout << i[j++];
      for(;j<11;j++)
	cout << setfill('0') << setw(8) << right << i[j];
      cout << "\n";
    }
  }
  BigInt operator+(const BigInt& rhs){
    BigInt result = rhs;
    for(int j=10;j>0;j--){
      result.i[j] += i[j];
      if(result.i[j]>=100000000){
	result.i[j]-=100000000;
	result.i[j-1]++;
      }
    }
    return result;
  }
};

int main(){
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string tmp;
    cin>>tmp;
    BigInt b(tmp);
    cin>>tmp;
    BigInt a(tmp);
    (a+b).print();
  }
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
int main(){
	int cnt, m;
	cin >> cnt;
	string s1, s2, ans;
	for(int i = 0; i < cnt; i++){
		cin >> s1 >> s2;
		ans = "";
		m = max(s1.size(), ans.size());
		while( s1.size() < m ) 
			s1 = '0' + s1;
		while( s2.size() < m )
			s2 = '0' + s2;		
		int c = 0;	
		for(int i = m-1; i >= 0; i--){
			char ch = ((s1[i] - '0') + (s2[i] - '0') + c) % 10 + '0';
			c = ((s1[i] - '0') + (s2[i] - '0') + c) / 10;
			ans += ch;
		}		
		if(c) 
			ans += c + '0';
		reverse(ans.begin(), ans.end());
		while(ans.size() > 1 && ans[0] == '0') 
			ans.erase(ans.begin());		
		if(ans.size() > 80)
			cout << "overflow" << endl;
		else
			cout << ans << endl;
	}
}

C++: #include <bits/stdc++.h>

using namespace std;

int main(){
        int N;
        cin>>N;
        for (int i = 0; i < N; i++) {
                string a,b,c,d;
                cin>>a;
                cin>>b;
                int x=max(a.length(),b.length());
                for (int j = 0; j < x-a.length()+1; j++) {
                        c+="0";
                }
                for (int j = 0; j < x-b.length()+1; j++) {
                        d+="0";
                }
                c+=a;
                d+=b;
                for (int j = 0; j < x+1; j++) {
                        c[j]+=d[j]-'0';
                }
                for (int j = x; j >= 0; j--) {
                        if(c[j]>'9'){
                                c[j]-=10;
                                c[j-1]++;
                        }
                }
                if(c[0]!='0')cout<<c[0];
                for (int j = 1; j <= x; j++) {
                        cout<<c[j];
                }
                cout<<endl;
        }

}