Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

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);

		
		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");
		printf("\n");
	}

	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<algorithm>

#define rep(i,n) for(int i=0;i<(n);i++)

using namespace std;

int main(){
	int T; scanf("%d",&T);
	while(T--){
		char s[128],t[128]; scanf("%s%s",s,t);
		int m=strlen(s),n=strlen(t);

		int a[128]={},b[128]={};
		rep(i,m) a[i]=s[m-i-1]-'0';
		rep(i,n) b[i]=t[n-i-1]-'0';

		m=max(m,n);
		int c[128]={};
		rep(i,m){
			int tmp=a[i]+b[i]+c[i];
			c[i]=tmp%10;
			c[i+1]+=tmp/10;
		}
		if(c[m]!=0) m++;

		if(m>80) puts("overflow");
		else{
			rep(i,m) putchar(c[m-i-1]+'0');
			puts("");
		}
	}

	return 0;
}

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

int main()
{
  int N;
  char str1[81];
  char str2[81];
  int j = 0, k = 0, c;
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    if(j > 80 || k > 80)
      cout << "overflow" << endl;
    else{
      int n1[41] = {};
      int n2[41] = {};
      int m1 = strlen(str1), m2 = strlen(str2);
      for(int i = 1, j = 40; (m1 - i - 1 >= 0) || (m1 - i >= 0); i += 2, j--)
	if(str1[m1 - i - 1] == '0')
	  if(str1[m1 - i] == '0')
	    n1[j] = 1000;
	  else 
	    n1[j] = str1[m1 - i] - '0';
	else
	  if(m1 - i - 1 < 0)
	    n1[j] = str1[m1 - i] - '0';
	  else
	    n1[j] = (str1[m1 - i] - '0') + (str1[m1 - i - 1] - '0') * 10; 
      for(int i = 1, j = 40; (m2 - i - 1 >= 0) || (m2 - i >= 0); i += 2, j--)
	if(str2[m2 - i - 1] == '0')
	  if(str2[m2 - i] == '0')
	    n2[j] = 1000;
	  else
	    n2[j] = (str2[m2 - i] - '0');
	else
	  if(m2 - i - 1 < 0)
	    n2[j] = str2[m2 - i] - '0';
	  else
	    n2[j] = (str2[m2 - i] - '0') + (str2[m2 - i - 1] - '0') * 10;
      int ans[41] = {};
      int i = 0;
      int m = (m1 > m2) ? m1 : m2;
      for(int j = 40; 39 - m / 2 < ((m % 2) ? j + 1 : j); i++, j--)
	if(n1[j] + n2[j] >= 100)
	  if(n1[j] == 1000 && n2[j] < 10){
	    n1[j - 1] *= 10;
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] < 10){
	    n2[j - 1] *= 10;
	    ans[i] = n1[j];
	  }else if(n1[j] == 1000 && n2[j] == 1000){
	    if(n1[j - 1] % 10 != 0)
	      n1[j - 1] *= 10;
	    ans[i] = 1000;
	  }else if(n1[j] == 1000 && n2[j] >= 10){
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] >= 10){
	    ans[i] = n1[j];
	  }else if(n1[j] + n2[j] < 110){
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] + 900;
	  }else{
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] - 100;
	  }
	else
	  ans[i] = n1[j] + n2[j];
      int k = i;
      for(; i > 0; i--)
	if((i == k) && (ans[i - 1] == 0))
	  ;
	else if(ans[i - 1] == 1000)
	  cout << "00";
	else if(ans[i - 1] > 1000)
	  cout << "0" << ans[i - 1] % 1000;
	else
	  cout << ans[i - 1];
      cout << endl;
    }
  }
}

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

int main(){
	int N;
	char str1[90], str2[90];
	int num1[80], num2[80], sum[80];
	int pos, pos2, up;
	
	scanf("%d", &N);
	for(int n = 0; n < N; n++){
		for(int i = 0; i < 90; i++){
			str1[i] = 0;
			str2[i] = 0;
		}
		for(int i = 0; i < 80; i++){
			num1[i] = 0;
			num2[i] = 0;
			sum[i] = 0;
		}
		scanf("%s", str1);
		scanf("%s", str2);
		
		pos = 89;
		while(pos >= 0 && str1[pos] == 0) pos--;
		pos2 = 0;
		while(pos >= 0 && pos2 < 80 && str1[pos] != 0){
			num1[pos2] = str1[pos] - 48;
			pos2++; pos--;
		}
		pos = 89;
		while(pos >= 0 && str2[pos] == 0) pos--;
		pos2 = 0;
		while(pos >= 0 && pos2 < 80 && str2[pos] != 0){
			num2[pos2] = str2[pos] - 48;
			pos2++; pos--;
		}
		
		/*for(int i = 0; i < 80; i++) printf("%d", num1[i]);
		printf("\n");
		for(int i = 0; i < 80; i++) printf("%d", num2[i]);
		printf("\n");*/
		
		up = 0;
		for(int i = 0; i < 80; i++){
			sum[i] = num1[i] + num2[i] + up;
			up = sum[i] / 10;
			sum[i] = sum[i] % 10;
		}
		
		if(up > 0 || sum[79] > 0) printf("overflow\n");
		else{
			pos = 79;
			while(pos >= 0 && sum[pos] == 0) pos--;
			if(pos < 0) printf("0");
			while(pos >= 0){
				printf("%d", sum[pos]);
				pos--;
			}
			printf("\n");
		}
	}
	
	return 0;
}