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<iostream>
#include<string>
#include<vector>
using namespace std;
int main() {
	int n,i,j,k,s,t,u;
	int carry = 0;
	string a, b,d;
	vector<int> c;
	cin >> n;
	for (i = 0; i < n; i++) {
		c.clear();
		cin >> a;
		cin >> b;
		while (a.size() > b.size()) {
			b = "0" + b;
		}
		while (b.size() > a.size()) {
			a = "0" + a;
		}
		s = a.size();
		for (j = s-1; j >=0; j--) {
			t = a[j]-'0' + b[j]-'0';
			if (carry == 1) {
				carry = 0;
				t += 1;
			}
			while (t >= 10) {
				t -= 10;
				carry += 1;
			}
			c.push_back(t);
		}
		while (carry == 1) {
			carry = 0;
			c.push_back(1);
		}
		u = c.size();
		if (u > 80||a.size()>80||b.size()>80) {
			cout << "overflow" << endl;
		}
		else if(u<=80){
			for (k = u - 1; k >= 0; k--) {
				cout << c[k];
			}
			cout << endl;
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>

using namespace std;

class longint
{
// friend
	friend ostream& operator<< (ostream&, const longint&);
	friend istream& operator>> (istream&, longint&);

// variable
private:
	string m_data;

// structor

public:
	longint(const string& data = "0") :
		m_data(data)
	{

	}

// operator
public:
	longint operator+(const longint& rhs) const
	{
		string data1 = m_data;
		string data2 = rhs.m_data;
		reverse(data1.begin(), data1.end());
		reverse(data2.begin(), data2.end());

		string result;
		int carry = 0;
		for(unsigned int i = 0; i < data1.length() || i < data2.length(); i++)
		{
			int num1 = 0;
			int num2 = 0;
			if(i < data1.length())
				num1 = ctoi( data1[i] );
			if(i < data2.length())
				num2 = ctoi( data2[i] );

			int sum = num1 + num2 + carry;
			result.push_back( itoc( sum % 10) );
			carry = sum / 10;
		}

		reverse(result.begin(), result.end());
		return longint(result);
	}

// method
private:
	inline static int ctoi(char c)
	{
		if(c < '0' || '9' < c)
			throw exception();

		return c - '0';
	}

	inline static char itoc(int i)
	{
		if(i < 0 || 9 < i)
			throw exception();

		return '0' + i;
	}
};

ostream& operator<< (ostream& stream, const longint& rhs)
{
	stream << rhs.m_data;
	return stream;
}

istream& operator>> (istream& stream, longint& rhs)
{
	stream >> rhs.m_data;
	return stream;
}

int main()
{
	longint result;

	int n;
	cin >> n;
	while(n-- > 0)
	{
		longint input;
		cin >> input;
		result = result + input;
	}

	cout << result << endl;

	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[90],b[90],ans[90];
	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<iostream>
#include<algorithm>
#include<string>


#define MAX 1000


using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

	for (int i = 0; i < n; i++){
		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;


		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		for (i = 0; out[i]; i++){
			;
		}

		if (i >= 81 || (flag == true && i >= 79)){
			cout << "overflow" << endl;
		}else{
			cout << out << endl;
		}
	}

	return 0;
}

C++: #include <stdio.h>

void rev_str(char str[1024]) {
	int length = 0;
	char str2[1024];
	int temp;

	int i;

	for(i = 0; i < 1024; i++)
		str2[i] = '\0';

	while(str[length])
		length++;

	temp = length;
	i = 0;
	while(temp) {
		str2[temp - 1] = str[i];
		i++;
		temp--;
	}

	for(i = 0; i < length; i++)
		str[i] = str2[i];
}

int exor(int a, int b) {
	if(a && b)
		return 0;
	else if(a || b)
		return 1;
	else
		return 0;
}

int main() {
	char input_a[1024];
	char input_b[1024];
	char ans[286][1024];
	char kuriagari;
	int temp;
	int n;

	int i, j;

	scanf("%d", &n);

	for(j = 0; j < n; j++) {
		for(i = 0; i < 1024; i++) {
			input_a[i] = '\0';
			input_b[i] = '\0';
			ans[j][i] = '\0';
		}

		kuriagari = 0;

		scanf("%s", input_a);
		scanf("%s", input_b);

		rev_str(input_a);
		rev_str(input_b);

		i = 0;
		while(input_a[i] && input_b[i]) {
			temp = input_a[i] + input_b[i] - 2 * '0' + kuriagari;
			ans[j][i] = temp % 10 + '0';
			kuriagari = temp / 10;
			i++;
		}

		if(kuriagari && !input_a[i] && !input_b[i]) {
			ans[j][i] = kuriagari + '0';
		}

		else {
			while (kuriagari && exor((int)input_a[i], (int) input_b[i])) {
				temp = input_a[i] + input_b[i] - '0' + kuriagari;
				ans[j][i] = (char)(temp % 10 + '0');
				kuriagari = temp / 10;
				i++;
			}

			if (kuriagari)
				ans[j][i] = kuriagari + '0';
		}

		while(input_b[i]) {
			ans[j][i] = input_b[i];
			i++;
		}

		while(input_a[i]) {
			ans[j][i] = input_a[i];
			i++;
		}

		rev_str(ans[j]);
		
		if(ans[j][80])
			printf("overflow");
		else
			printf("%s", ans[j]);

		putchar('\n');

	}

	return 0;
}