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 <string.h>
int main(void){
    int n,ap,bp,cp,t;
    char a[81],b[81],c[81];
    scanf("%d",&n);
    while(~scanf("%s%s",&a,&b)){
        ap=strlen(a);
        bp=strlen(b);
        cp=sizeof(c);
        c[cp]='\0';
        t=0;
        while(cp){
            t+=(ap?a[--ap]-'0':0)+(bp?b[--bp]-'0':0);
            c[--cp]='0'+(t%10);
            t/=10;
        }
        while(t&&cp>0) {
            c[--cp]='0'+(t%10);
        }
        if(t)printf("overflow\n");
        else printf("%s\n",&c[cp]);
    }
    return 0;
}

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

string calc(string s1,string s2){
  int num1=s1.size()-1,num2=s2.size()-1;
  int tmp=0;
  string s;
  while(!(num1==-1&&num2==-1&&tmp==0)){
    if(num1!=-1&&num2!=-1){
      tmp+=int(s1[num1]-'0')+int(s2[num2]-'0');
      num1--;num2--;
    }else if(num1!=-1&&num2==-1){
      tmp+=int(s1[num1]-'0');
      num1--;
    }else if(num1==-1&&num2!=-1){
      tmp+=int(s2[num1]-'0');
      num2--;
    }
    s=to_string(tmp%10)+s;
    tmp/=10;
  }
  if(s.size()>80)s="overflow";
  return s;
}

int main(){
  cin.tie(0);ios::sync_with_stdio(false);
  int n;
  string s1,s2;

  cin>>n;
  for(int i=0;i<n;i++){
    cin>>s1>>s2;
    if(s1.size()>80||s2.size()>80){
      cout<<"overflow"<<endl;
      continue;
    }
    
    cout<<calc(s1,s2)<<endl;
  }
  return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string>
#include <iostream>
#include <math.h>
#include <stack>

using namespace std;

int main(){
    
    string d[100000];
    string e[100000];
    
    stack<char> st1;
    stack<char> st2;
    stack<int> st3;
    int n=0;
    scanf("%d",&n);
    for(int i=0; i<n; i++){
        cin>>d[i];
        cin>>e[i];
    }
    
    for(int i=0; i<n; i++){
        //cin>>d;
        //cin>>e;
        
        for(int j=0; j<(int)(d[i].size()); j++){
            st1.push(d[i][j]);
        }
        for(int j=0; j<(int)(e[i].size()); j++){
            st2.push(e[i][j]);
        }
        int temp=0;
        char temp_char;
        while(st1.empty()==false||st2.empty()==false){
            if(st1.empty()==false&&st2.empty()==false){
                temp_char=st1.top();
                temp+=atoi(&temp_char);
                temp_char=st2.top();
                temp+=atoi(&temp_char);
                st1.pop();
                st2.pop();
            }else if(st1.empty()==false&&st2.empty()==true){
                temp_char=st1.top();
                temp+=atoi(&temp_char);
                st1.pop();
            }else if(st1.empty()==true&&st2.empty()==false){
                temp_char=st2.top();
                temp+=atoi(&temp_char);
                st2.pop();
            }
            st3.push(temp%10);
            temp=temp/10;
        }
        
        if(temp!=0){
            st3.push(temp);
        }
        
        if(st3.size()>80){
            printf("overflow");
        }else{
            while(st3.empty()==false){
                printf("%d",st3.top());
                st3.pop();
            }
        }
        printf("\n");
        
        
    }
    
}

C++: #include<bits/stdc++.h>
using namespace std;

string solve(string s1, string s2){
while(s1.size()<85){
s1 = '0' + s1;
}
while(s2.size()<85){
s2 = '0' + s2;
}
string ret = "";
int kuri = 0;
for(int i=0;i<80;i++){
int r = (int)(s1[s1.size()-1-i]-'0') + (int)(s2[s2.size()-1-i]-'0') + kuri;
kuri = r/10;
r = r%10;
ret = ((char)('0'+r)) + ret;
}
if (kuri>0) return "overflow";
else{
string ret2 = "0";
for(int i=0;i<ret.size();i++){
if (ret[i]!='0'){ret2 = ret.substr(i);break;}
}
return ret2;
}
}

int main(void){
int N;
cin >> N;
for(int i=0;i<N;i++){
string s1, s2;
cin >> s1 >> s2;
cout << solve(s1, s2) << endl;
}
return 0;
}

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

#define NUMBER_DIGITS 80

int main()
{
  int N;
  char str1[NUMBER_DIGITS + 1];
  char str2[NUMBER_DIGITS + 1];
  char str3[NUMBER_DIGITS / 2 + 1][3];
  char str4[NUMBER_DIGITS / 2 + 1][3];
  int n1[NUMBER_DIGITS / 2 + 1] = {};
  int n2[NUMBER_DIGITS / 2 + 1] = {};
  int j = 0, k = 0, c;
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > NUMBER_DIGITS || m2 > NUMBER_DIGITS)
      cout << "overflow" << endl;
    else{
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m1 + 1; i += 2, j--){
	if(m1 % 2 == 1 && m1 - i < 0){
	  str3[j][0] = str1[0];
	  str3[j][1] == '\0';
	}else{
	  strncpy(str3[j], &str1[m1 - i], 2);
	  str3[j][2] == '\0';
	}
      }
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m2 + 1; i += 2, j--){
	if(m2 % 2 == 1 && m2 - i < 0){
	  str4[j][0] = str2[0];
	  str4[j][1] == '\0';
	}else{	  
	  strncpy(str4[j], &str2[m2 - i], 2);
	  str4[j][2] == '\0';
	}
      }
      for(int i = NUMBER_DIGITS / 2; i + ((m1 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m1 / 2; i--)
	if(str3[i][0] == '0' && str3[i][1] == '0')
	  n1[i] = 1000;
	else
	  n1[i] = atoi(str3[i]);
      for(int i = NUMBER_DIGITS / 2; i + ((m2 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m2 / 2; i--)
	if(str4[i][0] == '0' && str4[i][1] == '0')
	  n2[i] = 1000;
	else
	  n2[i] = atoi(str4[i]);
      int ans[NUMBER_DIGITS / 2 + 1] = {};
      int i = 0;
      int m = (m1 > m2) ? m1 : m2;
      for(int j = NUMBER_DIGITS / 2; NUMBER_DIGITS / 2 - 1 - 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;
    }
  }
}