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() {

	string a;
	string b;
	int kari;

	cin >> kari;

	for (int o = 0; o < kari; o++) {
		int c[80] = {0};
		int d[80] = {0};
		int e[81] = {0};
		int counter;

		cin >> a;
		cin >> b;
	
		if (a.size() > 80 || b.size() > 80) {
			cout << "overflow" << endl;
			continue;
		} else {
			for (int i = a.size() - 1; i >= 0; i--) {
				c[a.size() - 1 - i] = a[i] - '0';
			}

			for (int i = b.size() - 1; i >= 0; i--) {
				d[b.size() - 1 - i] = b[i] - '0';
			}

			for (int j = 0; j < 80; j++) {
				e[j] += c[j] + d[j];
				if (e[j] >= 10) {
					e[j+1] = e[j] / 10;
					e[j] = e[j] % 10;
				} 
			}
		}

		if (e[80] > 0) {
			cout << "overflow" << endl;
		} else {
			for (int k = 79; k >= 0; k--) {
				if (e[k] != 0) {
					counter = k;
					break;
				}
			}
			for (int k = counter; k >= 0; k--) {
				cout << e[k];
			}

			cout << endl;
		}
	}
	return 0;
}

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

int main() {
	int dsn; scanf("%d", &dsn);
	for(int dsi = 0; dsi < dsn; dsi++) {
		static char ais[90],bis[90];
		scanf(" %s %s",ais,bis);
		int aislen = strlen(ais);
		int bislen = strlen(bis);
		static int ad[90],bd[90];
		fill(ad,ad+90,0);
		fill(bd,bd+90,0);
		for(int i = 0; i < aislen; i++) ad[i] = ais[aislen-1-i]-'0';
		for(int i = 0; i < bislen; i++) bd[i] = bis[bislen-1-i]-'0';
		int cl = 0;
		for(int i = 0; i < 90; i++) {
			ad[i] += bd[i]+cl;
			cl = ad[i]/10;
			ad[i] %= 10;
		}
		bool flag = false;
		for(int i = 89; i >= 0; i--) {
			flag = flag || ad[i];
			if(flag) {
				printf("%d",ad[i]);
			}
		}
		if(!flag)printf("0");
		printf("\n");
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#define MAX 80

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] -= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}

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

C++: #include<stdio.h>
#include<iostream>
#include<list>
#include<map>
#include<vector>
#include<algorithm>
#include<string.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << x << endl;
using namespace std;

int main(){
    int n;
    cin >> n;
    rep(i,n){
        char input1[81], input2[81];
        cin >> input1 >> input2;
        //後ろから和を求める
        int ans[81];
        int size1 = strlen(input1) - 1;
        int size2 = strlen(input2) - 1;
        int i = 0;
        while(1){
            if(size1 - i >= 0 && size2 - i >= 0)
                ans[i] = input1[size1 - i] - '0' + input2[size2 - i] - '0';
            else if(size1 - i >= 0)
                ans[i] = input1[size1 - i] - '0';
            else if(size2 - i >= 0)
                ans[i] = input2[size2 - i] - '0';
            else 
                break;
            i++;
        }
        //桁数を繰り上げる
        rep(j, i - 1){
            if(ans[j] >= 10){
                ans[j] -= 10;
                if(ans[j + 1] >= 18)
                    ans[j + 1] = 1;
                else
                    ans[j + 1]++;
            }
        }
        //出力
        if(ans[80] == 1)
            cout << "overflow" << endl;
        else{
            for(int j = i - 1; j >= 0; j--)
                cout << ans[j];
        }
        cout << endl;
    }
}

C++: #include<iostream>
#include<string>
#include<vector>
using namespace std;
int main(){
	string a,b;
	vector<int> ans;
	int n,tmp,carry,num;
	bool flag = true;
	cin >> n;
	
	for(int i=0; i<n; i++){
		ans.clear();
		cin >> a >> b;
		int ai = a.size()-1;
		int bi = b.size()-1;
		
		if(ai < 0 && bi < 0 && carry==0){
			cout << "overflow";
			flag = false;
		}
		carry = 0;
		while(ai >= 0 || bi >= 0){
			tmp=0;
			if(ai >= 0) tmp += (a[ai] - '0');
			if(bi >= 0) tmp += (b[bi] - '0');
			ans.push_back((tmp+carry)%10);
			carry = (tmp+carry)/10;
			ai--; bi--;
		}
		
		if(carry != 0) ans.push_back(carry);
		
		if(flag == true){
		
			if(ans.size() > 80){
				cout << "overflow";
				//flag = false;
			}
		
			else{
				for(int j=ans.size()-1; j>=0; j--)
					cout << ans[j];
			}
			
		}
		
	cout << endl;
	
	}
	
	return 0;
}

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

using namespace std;

int main() {
    int N;
    cin >> N;
    while(N--) {
        string s1, s2;
        cin >> s1 >> s2;

        char buf[81] = {0};
        int p1 = s1.size()-1, p2 = s2.size()-1, pb = 79;
        int carry = 0;
        while(p2 >= 0) buf[pb--] = s2[p2--];
        pb = 79;
        while(p1 >= 0) {
            int bd = buf[pb];
            if(bd > 0) bd -= '0';
            int d = s1[p1--]-'0' + bd + carry;
            carry = d / 10;
            d %= 10;
            buf[pb--] = d+'0';
        }
        if(pb == -1 && carry > 0) cout << "overflow" << endl;
        else {
            if(carry > 0) buf[pb--] = carry+'0';
            cout << (char*)(buf+(pb+1)) << endl;
        }
    }
    return 0;
}