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<math.h>
#include<string.h>

int main(void){

	int N,math1,math2,kuri,keta;
	int i,j,k;
	char fi[100],se[100],ans[100];

	scanf("%d",&N);

	for(i=0;i<N;i++){

		scanf("%s",&fi);
		scanf("%s",&se);

		if(strlen(fi)<=80&&strlen(se)<=80){
		for(j=strlen(fi)-1,k=strlen(se)-1,keta=0,kuri=0;j>=0||k>=0;j--,k--,keta++){
			if(j<0) math1=0;
			else math1=fi[j]-'0';
			if(k<0) math2=0;
			else math2=se[k]-'0';
            /*ans[keta]=((math1+math2+kuri)%10)+'0';*/
			kuri=(math1+math2+kuri)/10;
		}
		if(kuri==1) printf("1");
		if(keta-1<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		}
		else printf("overflow");
		printf("\n");
	}

	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)
  {
    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);

    auto it_a = this->data.begin ();
    auto it_b = obj.data.begin ();

    while (it_a != this->data.end ())
      {
	*it_a += *it_b;
	auto bufit = it_a;
	for (int i = 0; i < this->data.size () - 1; i++)
	  {
	    if (*bufit >= 10)
	      {
		bufit--;
		*bufit += 1;
		++bufit;
		*bufit -= 10;
		bufit--;
	      }
	  }
	it_a++;
	it_b++;
      }
    it_a = this->data.begin ();
    if (*it_a == 0)
      this->data.pop_front ();
    return *this;
  }
};

int
main ()
{
  string s1, s2;
  int loop;
  cin >> loop;
  while (loop-- > 0)
    {
      cin >> s1 >> s2;
      bigdata data1, data2;
      data1 = s1;
      data2 = s2;
      data1 = data1 + data2;
      if ((data1.to_str().length()) > 80)
	cout << "overflow" << endl;
      else
	cout << data1.to_str () << endl;
    }
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[1000],b[1000],ans[1000];
	int f;
	int alen,blen,mlen;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		alen=in.size();
		blen=in2.size();
		for(int i=0;i<alen;i++){
			a[alen-1-i]=in[i]-'0';
		}
		for(int i=0;i<blen;i++){
			b[blen-1-i]=in[i]-'0';
		}
		mlen=max(alen,blen);
		if(mlen>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<mlen+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[mlen]!=0){
			//if(mlen>79){
				//cout<<"overflow"<<endl;
				//continue;
			//}
			//cout<<ans[mlen];
		}
		
		for(int i=0;i<mlen;i++){
			cout<<ans[mlen-1-i];
		}cout<<endl;
		
		
	}
	return 0;
}

C++: #include <stdio.h>

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	int num1[81];
	int num2[81];
	int sum[81];
	char str[82];
	int n;
	int i, j, k;
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<81; j++) {
			num1[j] = num2[j] = 0;
		}
		for (j=0; j<81; j++) {
			sum[j]=0;
		}		
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num1[k++] = (int)(str[j] - '0'); 
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num2[k++] = (int)(str[j] - '0'); 
		
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j] >= 10) {
				sum[j+1] += 1;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] != 0) {
			printf("overflow\n");
		}
		else {
			j=79;
			while (sum[j] == 0)  j--;
			for (; j>=0; j--) {
				printf("%d", sum[j]);
			}
			printf("\n");
		}
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <string>
#include <algorithm>
#include <string>
#include <climits>

#define REP(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)
#define REG(i, a, b) for (int (i) = ((int)(a)); (i) < ((int)(b)); i++)
#define ALL(V) (V).begin(), (V).end()
#define PRINT(STR) cout << (STR) << endl

typedef long long ll;
using namespace std;

double nextDouble() {
	double d = -1;
	cin >> d;
	return d;
}

int nextInt() {
	int n = -1;
	cin >> n;
	return n;
}

string nextString() {
	string str;
	cin >> str;
	return str;
}

string reverse(string str) {
	string t = str;
	for (int i = 0; i < str.size(); i++) {
		t[i] = str[str.size() - i - 1];
	}
	return t;
}

int main() {
	cin.tie(0); //対話型には使えない
	int n = nextInt();
	while (n--) {
		string str1 = nextString();
		string str2 = nextString();
		int n[100];
		fill(n, n + 100, 0);
		str1 = reverse(str1);
		str2 = reverse(str2);

		REP(i, str1.size()) {
			n[i] = str1[i] - '0';
		}
		REP(i, str2.size()) {
			n[i] += str2[i] - '0';
		}
		if (str1.size() >= 80 || str2.size() >= 80) {
			PRINT("overflow");
			continue;
		}
		int M = max(str1.size(), str2.size());
		int k = M;
		REP(i, M) {
			if (n[i] >= 10) {
				n[i + 1] += n[i] / 10;
				n[i] %= 10;
				if (i == M - 1) k = M + 1;
			}
		}
		if (n[80] > 0) {
			PRINT("overflow");
			continue;
		}
		for (int i = k - 1; i >= 0; i--) {
			cout << n[i];
		}
		cout << endl;
		
	}
	
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int N;cin>>N;
  for(int i=0;i<N;i++){
    string num1,num2;
    cin>>num1>>num2;
    if(num1.size()==num2.size()&&num1.size()>80||num2.size()>80){
      cout<<"overflow"<<endl;
      continue;
    }
    int ma,mi;
    stack<int> sum;
    string lnum,snum;
    if(num1.size()>num2.size()){
      ma=num1.size();lnum=num1;
      mi=num2.size();snum=num2;
    }else{
      ma=num2.size();lnum=num2;
      mi=num1.size();snum=num1;
    }
    int keta=0;
    for(int i=ma-1;i>=0;i--){
      int a,b;
      a=lnum[i]-'0';
      if(mi<=0)
	b=0;
      else{
	b=snum[mi-1]-'0';
	mi--;
      }
      int summ=a+b+keta;
      if(summ>=10){
	sum.push(summ%10);
	keta=1;
      }else if(summ<=9){
	sum.push(summ);
	keta=0;
      }
    }
    if(keta==1)
      sum.push(1);
    if(sum.size()>80)
      cout<<"overflow"<<endl;
    else{
      int k=sum.size();
      for(int i=0;i<k;i++){
	cout<<sum.top();
	sum.pop();
      }
      cout<<endl;
    }
  }
  return 0;
}