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++: #define scanf_s scanf
#define sscanf_s sscanf
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 81
#define _MAX 100
//#define CONSTNUM 10000000
int main(void)
{
	int n, s = 0, x, y, r = 0, a, b;
	int sum_n[50][3] = { 0 };
	char sum[50][_MAX], astr[_MAX], bstr[_MAX], num[11] = { "0123456789" };
	char ca, cb;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%s %s", &astr,_MAX, &bstr, _MAX);
		for (x = 0; astr[x] != '\0'; ++x);
		for (y = 0; bstr[y] != '\0'; ++y);
		if (x > 80 || y > 80) {
			printf("overflow\n");
			continue;
		}
		for (int j = 0; j < x || j < y; ++j) {
			if (j < x && j < y) {
				ca = astr[x - j - 1]; cb = bstr[y - j - 1];
				sscanf_s(&ca, "%d", &a);
				sscanf_s(&cb, "%d", &b);
				if (x > y) sum[i][x - j] = num[(a + b + r) % 10];
				else sum[i][y - j] = num[(a + b + r) % 10];
				if (a + b + r >= 10) r = (a + b + r) / 10;
				else r = 0;
			}
			else {
				if (x > y) {
					if (r != 0) {
						ca = astr[x - j - 1];
						sscanf_s(&ca, "%d", &a);
						sum[i][x - j] = num[(a + r) % 10];
						if (a + r >= 10) r = (a + r) / 10;
						else r = 0;
					}
					else {
						sum[i][x - j] = astr[x - j - 1];
					}
				}
				else {
					if (r != 0) {
						cb = bstr[y - j - 1];
						sscanf_s(&cb, "%d", &b);
						sum[i][y - j] = num[(b + r) % 10];
						if (b + r >= 10) r = (b + r) / 10;
						else r = 0;
					}
					else {
						sum[i][y - j] = astr[y - j - 1];
					}
				}
			}
		}
		sum_n[i][0] = x; sum_n[i][1] = y; sum_n[i][2] = r;
		r = 0;
	}
	for (int i = 0; i < n; ++i) {
		x = sum_n[i][0]; y = sum_n[i][1]; r = sum_n[i][2];
		if (y <= 80 - r && x <= 80 - r) {
			if (r == 1) printf("%d", r);
			for (int j = 1; j <= x || j <= y; ++j) {
				printf("%c", sum[i][j]);
			}
		}
		else {
			printf("overflow");
		}
		printf("\n");
	}
}

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

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		if( num1.size() > 80 || num2.size() > 80 ){
			cout << "overflow" << endl;
			continue;
		}
		int sum[101];
		int degit = 0;
		for( int i = 0; i < 101; i++ ) sum[i] = 0;
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1]-'0';
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			if( i < num2.size() ){
				now = sum[100-i] + (int)num2[num2.size()-i-1]-(int)('0') + degit;
			}else{
				now = sum[100-i] + degit;
			}
			sum[100-i] = (now%10);
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		if( d < 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}

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

#define rep2(x,from,to) for(int x=(from);(x)<(to);(x)++)
#define rep(x,to) rep2(x,0,to)
int main() {
	int n;
	cin >> n;
	while(n--) {
		char a[84], b[84];
		int x[84], y[84], s[84], max;
		cin >> a >> b;
		int i = 0, j = 0;
		for(; a[i] != '\0'; ++i) {
			x[i] = a[i] - '0';
		}
		for(; b[j] != '\0'; ++j) {
			y[j] = b[j] - '0';
		}
		if(i > j) {
			for(int k = j - 1; k >= 0; k--) {
				y[i-j+k] = y[k];
			}
			rep(k,i-j) {
				y[k] = 0;
			}
			max = i;
		} else if(i < j) {
			for(int k = i - 1; k >= 0; k--) {
				x[j-i+k] = x[k];
			}
			rep(k,j-i) {
				x[k] = 0;
			}
			max = j;
		}
		bool f = 0;
		for(int k = max - 1; k >= 0; k--) {
			int c = x[k] + y[k];
			if(f) c++;
			if(c >= 10) {
				f = 1, c -= 10;
			} else f = 0;
			s[k] = c;
		}
		if(f) {
			max++;
		}
		if(max > 80) cout << "overflow" << endl;
		else {
			if(f) {
				cout << "1";
				max--;
			}
			rep(k,max) {
				cout << s[k] + '0';
			}
			cout << endl;
		}
	}
	return 0;
}

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

int main()
{
  int N;
  char str1[81];
  char str2[81];
  char s1[81];
  char s2[81];
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > 80 || m2 > 80)
      cout << "overflow" << endl;
    else{
      if(m1 > m2){
	for(int i = 0; i < m1 - m2 + 1; i++)
	  s1[i] = '0';
	s1[m1 - m2] = '\0';
	strcat(s1, str2);
	strcpy(s2, str1);
      }else if(m2 > m1){
	for(int i = 0; i < m2 - m1; i++)
	  s2[i] = '0';
	s2[m2 - m1] = '\0';
	strcat(s2, str1);
	strcpy(s1, str2);
      }else{
	strcpy(s1, str2);
	strcpy(s2, str1);
      }
      int ans[81];
      for(int i = ((m1 > m2) ? m1 : m2) - 1; i >= 0; i--)
	if((s1[i] - '0') + (s2[i] - '0') >= 10){
	  s1[i - 1] = s1[i - 1] + 1;
	  ans[i] = (s1[i] - '0') + (s2[i] - '0') - 10;
	}else
	  ans[i] = (s1[i] - '0') + (s2[i] - '0');
      if((s1[0] - '0') + (s2[0] - '0') >= 10)
     	cout << "1";
      for(int i = 0; i < ((m1 > m2) ? m1 : m2); i++)
	cout << ans[i];
    }
    cout << endl;
  }
}

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

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		if( num1.size() > 80 || num2.size() > 80 ){
			cout << "overflow" << endl;
			continue;
		}
		int sum[101];
		int degit = 0;
		for( int i = 0; i < 101; i++ ) sum[i] = 0;
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1]-'0';
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			if( i < num2.size() ){
				now = sum[100-i] + (int)num2[num2.size()-i-1]-(int)('0') + degit;
			}else{
				now = sum[100-i] + degit;
			}
			sum[100-i] = (now%10);
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		if( d <= 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}

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

int main(){
	int n,t,i;
	char a[100],b[100],x[100],y[100];
	scanf("%d",&n);
	for(int o=0;o<n;o++){
		t=0;
		scanf("%s %s",a,b);
		for(i=0;a[i]!='\0';i++){}
		for(int j=0;j<i;j++){
			x[j]=a[i-j-1];
		}
		for(;i<100;i++){
			x[i]='0';
		}
		for(i=0;b[i]!='\0';i++){}
		for(int j=0;j<i;j++){
			y[j]=b[i-j-1];
		}
		for(;i<100;i++){
			y[i]='0';
		}
		for(i=0;i<99;i++){
			x[i]+=y[i]-'0';
			if(x[i]>'9'){
				x[i]-=10;
				x[i+1]+=1;
			}
		}
		for(i=99;i>=0;i--){
			if(x[i]!='0' || t){
				if(i>79){
					printf("overflow\n");
					break;
				}
				printf("%c",x[i]);
				t=1;
			}
		}
		printf("\n");
	}
	return 0;
}