Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

C++: #include<stdio.h>
#include<iostream>
#include<string>
using namespace std;
int main(){

int n;
cin>>n;
for(int iii=0;iii<n;iii++){
string st1,st2;
int n1[83]={},n2[83]={};
cin>>st1>>st2;
for(int i=0;i<st1.size();i++)
n1[i]=st1[st1.size()-i-1]-'0';
for(int i=0;i<st2.size();i++)
n2[i]=st2[st2.size()-i-1]-'0';
int N[83]={};int y=0;

int r=st1.size();
if(r<st2.size())r=st2.size();
for(int i=0;i<r;i++){
N[i]=(n1[i]+n2[i]+y)%10;
y=(n1[i]+n2[i]+y)/10;
if(i==r-1&&y>0)r++;
}
if(r>81)cout<<"overflow"<<endl;
else{
for(int i=0;i<r;i++)
cout<<N[r-i-1];
cout<<endl;
}}
return 0;
}

C++: #include <iostream>
#include <cmath>

using namespace std;

int string_to_d(const string &s){
	int num = 0;
	if(!s.empty())
	for(int i=s.size()-1;i>=0;--i){
		if('1'<=s[i]&&s[i]<='9') num += (s[i]-'0')*pow(10,s.size()-i-1);
	}
	return num;
}

int main(void){
	int n;
	while(cin >> n){
		for(int a=0;a<n;++a){
			string s1,s2;
			cin >> s1 >> s2;
			int ex1num[10];//8*10=80桁
			int ex2num[10];
			int sum[10] = {0};
			for(int i=s1.size();i<80;++i) s1="0"+s1;
			for(int i=s2.size();i<80;++i) s2="0"+s2;
			for(int i=0;i<10;++i){
				sum[i] = 0;
				ex1num[i] = string_to_d(s1.substr((10-i-1)*8,8));
				ex2num[i] = string_to_d(s2.substr((10-i-1)*8,8));
			}
			int carry = 0;
			for(int i=0;i<10;++i){
				sum[i] = (ex1num[i]+ex2num[i]+carry)%(int)1e+008;
				carry = floor((ex1num[i]+ex2num[i]+carry)/(int)1e+008);
			}
			if(carry>0){
				cout << "overflow\n";
				continue;
			}
			bool zeroshow = false;
			for(int i=9;0<=i;--i){
				if(zeroshow&&sum[i]!=0) for(int j=0;j<7-(int)log10(sum[i]);j++) cout << "0";
				if(zeroshow&&sum[i]==0) for(int j=0;j<8;j++) cout << "0";
				if(sum[i]!=0) cout << sum[i];
				if(sum[i]>0) zeroshow = true;
			}
			cout << "\n";
		}
	}
	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(itr,c) 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;
  int len1, len2;
  char tmp;

  scanf("%d",&n);
  rep(i,n){
    string str1, str2, res;
    cin >> str1;
    cin >> str2;

    vector<int> a(82, 0), b(82, 0);
    len1 = str1.size(); len2 = str2.size();
    rep(j,len1) a[j] = str1[len1-1-j] - '0';
    rep(j,len2) b[j] = str2[len2-1-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());
    int f = 0;
    rep(j,res.size()){
      if(res[j] != '0') f = 1;
      if(f) cout << res[j];
    }

    cout << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <vector>
#include <string>
#include <queue>
#include <map>
#include <algorithm>
#include <set>
#define INF 10000000
using namespace std;
typedef pair<int,int> P;

int main(){
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string str1,str2;
    cin >> str1 >> str2;
    if(str1.size()>str2.size()){
      string temp = str1;
      str1 = str2;
      str2 = temp;
    }

    int len1 = str1.size(),len2 = str2.size();
    int kuriagari = 0;
    for(int j=1;len2-j>=0;j++){
      if(len1-j>=0){
        int sum = (str2[len2-j]-'0') + (str1[len1-j]-'0') + kuriagari;
        if(sum>=10){
          sum -=10;
          kuriagari = 1;
        }else{
          kuriagari = 0;
        }
        str2[len2-j] = sum+'0';
      }else{
        if(kuriagari){
          int sum = (str2[len2-j]-'0') + kuriagari;
          if(sum>=10){
            sum -=10;
            kuriagari = 1;
          }else{
            break;
          }
          str2[len2-j] = sum+'0';
        }else{
          break;
        }
      }
    }
    if(kuriagari){
      str2 = "1" + str2;
    }
    if(str2.size()>80){
      cout << "overflow" << endl;
    }else{
      cout << str2 << endl;
    }
  }
}

C++: #include <iostream>
#include <cstring>

using namespace std;

int A[1000],B[1000],C[1000];

int main()
{
    int n;
    cin >> n;
    while(n > 0)
    {
        n--;
        string str1,str2;
        cin >> str1 >> str2;
        int len1 = str1.length();
        int len2 = str2.length();
        if(len1 < len2)
        {
            string st = str1;
            str1 = str2;
            str2 = st;
            int ll = len1;
            len1 = len2;
            len2 = ll;
        }
        for(int i = 0; i < 1000; i++)
        {
            C[i] = 0;
            A[i] = 0;
            B[i] = 0;
        }

        for(int i = 0; i < len1; i++)
            A[i] = str1[len1 - 1 - i] - '0';
        for(int i = 0; i < len2; i++)
            B[i] = str2[len2 - 1 - i] - '0';
        for(int i = 0 ; i < len1; i++)
        {
            C[i] = A[i] + B[i] + C[i];
            C[i+1] = C[i] / 10;
            C[i] = C[i] % 10;
        }
        //for(int i = len2; i <len1; i++)
       // {
           //  C[i] = C[i] + A[i];
           //  C[i+1] = C[i] / 10;
          //   C[i] = C[i] % 10;
       // }


        if(C[80] > 0)
            cout << "overflow" << endl;
        else
            {
                if(C[len1] > 0)
                    len1++;
                for(int i = len1-1; i >= 0; i--)
                    cout << C[i];
                cout << endl;
            }
    }
    return 0;
}

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

int main(){
	int n;
	cin>>n;
	for(int i=0; i<n; i++){
		string a, b;
		cin>>a>>b;
		int kuri = 0;
		int k = 0;
		string ans = "";
		while(true){
			bool breakflg = true;
			int ai = 0;
			int bi = 0;
			if(int(a.size())-k-1 >= 0){
				ai = a[int(a.size())-k-1] - '0';
				breakflg = false;
			}
			if(int(b.size())-k-1 >= 0){
				bi = b[int(b.size())-k-1] - '0';
				breakflg = false;
			}
			k++;
			int sum = ai + bi + kuri;
			if(sum>9){
				kuri = 1;
				sum %= 10;
			} else {
				kuri = 0;
			}
			ans.push_back('0'+sum);
			if(!kuri&&breakflg) break;
		}
		reverse(ans.begin(), ans.end());
		while(true){
			if(ans.size()==1) break;
			if(ans[0]=='0'){
				ans = ans.substr(1);
			} else {
				break;
			}
		}
		cout<<ans<<endl;
	}
	return 0;
}