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 <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int main(){
  int n, maxlen;
  char a[101], b[101];

  cin >> n;
  for(int ii = 0 ; ii < n ; ii++){
    string ans = "";
    scanf("%s%s", a, b);
    
    maxlen = max(strlen(a), strlen(b));

    reverse(a, a+strlen(a));
    reverse(b, b+strlen(b));
    
    for(int i = strlen(a) ; i < 100 ; i++){
      a[i] = '0';
    }
    for(int i = strlen(b) ; i < 100 ; i++){
      b[i] = '0';
    }

    int tmp, tmp2 = 0;
    bool flag = 0;

    for(int i = 0 ; i < maxlen ; i++){
      tmp = (a[i] - '0') + (b[i] - '0') + tmp2;
      
      if(i == maxlen - 1){
	if(tmp >= 10){
	  tmp2 = 1;
	  tmp -= 10;
	  ans += tmp + '0';
	  ans += '1';
	}
	else ans += tmp + '0';
      }

      else if(tmp >= 10){
	tmp2 = 1;
	tmp -= 10;
	ans += tmp + '0';
      }
      else if(tmp <= 9){
	ans += tmp + '0';
	tmp2 = 0;
      }
    }

    reverse(ans.begin(), ans.end());

    int size = ans.size();

    if(size >= 80 || maxlen >= 80) cout << "overflow" << endl;
    else cout << ans << endl;
  }
  return 0;
}

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

int main(void)
{
int N,j,k=0;
cin >> N;
char strA[100000],strB[100000];
while(1){
k++;
if(k==N+1){
break;
}
cin >> strA >> strB;
int ans[81]={0};
ans[0]=11111;
if(strlen(strA)>80||strlen(strB)>80){
cout << "overflow" << endl;
continue;
}
int a[81]={0},b[81]={0};
if(strA[strlen(strA)-1]=='0'&&strB[strlen(strB)-1]=='0'){
    cout << 0 << endl;
    continue;
}
j=strlen(strA)-1;
for(int i=80 ; j>=0 ; i--){
a[i]=(int)strA[j]-48;
j--;
}
j=strlen(strB)-1;
for(int i=80 ; j>=0 ; i--){
b[i]=(int)strB[j]-48;
j--;
}
for(int i=80 ; i!=0 ; i--){
ans[i]=ans[i]+a[i]+b[i];
if(ans[i]>=10){
ans[i]=ans[i]-10;
ans[i-1]++;
}
}
if(ans[0]!=11111){
cout << "overflow" << endl;
continue;
}
for(int i=1 ; i<81 ; i++){
if(ans[i]!=0){
j=i;
break;
}
}
char c[3];
for(int i=j ; i<81 ; i++){
printf("%c",ans[i]+48);
}
cout << '\n';
}
return 0;
}

C++: int add(char *retstr, char *str1, char *str2){

	int str1len=strlen(str1);
	int str2len=strlen(str2);
	char ad,i1,i2;
	char offset1,offset2;
	char c1,c2;
	bool carry=false;

	if(str1len>str2len){
		offset1=0;
		offset2=str1len-str2len;
	}else{
		offset1=str2len-str1len;
		offset2=0;
	}

	for(int i=max(str1len,str2len);i<82;++i){
		retstr[i]=0;
	}

	for(int i=max(str1len,str2len)-1;i>=0;--i){

		if(i-offset1>=0){c1=str1[i-offset1];}else{c1=48;}
		if(i-offset2>=0){c2=str2[i-offset2];}else{c2=48;}
		i1=c1-48;i2=c2-48;//Ascii code to Number

		ad=i1+i2;
		if(carry)++ad;//桁上がり分

		if(ad>=10){ad %= 10; carry=true;}else{carry=false;}
		retstr[i+1]=ad+48;
	}

	if(carry){
		retstr[0]=1+48;
	}else{
		retstr[0]=0+48;
		for(int i=0;i<strlen(retstr)-1;++i){
			retstr[i]=retstr[i+1];
		}
		retstr[strlen(retstr)-1]=0;
	}

	if(strlen(retstr)>=81)return 1;

	return 0;

}

int main(){

	int n,overflowed;
	char str1[82], str2[82], retstr[82];

	cin>>n;

	for(int i=0;i<n;++i){

		cin>>str1;
		cin>>str2;

		if(strlen(str1)>=81||strlen(str2)>=81){
			cout<<"overflow"<<endl;
		}else{
			overflowed=add(retstr, str1, str2);
			if(overflowed){
				cout<<"overflow"<<endl;
			}else{
				cout<<retstr<<endl;
			}
		}
	}

	return 0;
}

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

int main(){
	string a, b;
	vector<int> ans;
	int n, num, ai, bi, c=0;
	bool flag;

	cin >> n;
	
	for(int i=0 ; i<n ; i++){
		cin >> a >> b;
		flag = false;
		
		for(int j=0 ; j<100 ; j++){
			num = 0;
			ai = a.size()-j-1;
			bi = b.size()-j-1;
			if( ai<0 && bi<0 && c==0) break;
			
			if(ai>=0)num += a[ ai ] - 48;
			if(bi>=0)num += b[ bi ] - 48;
			ans.push_back( (num+c)%10 );
			if(num+c >= 10) c = 1;
			else c = 0;
		}
		if(ans.size()>80){
			flag = true;
		}
		if(flag){
			printf("overflow");
		}else{
			for(int j=ans.size()-1 ; j>=0 ; --j){
				printf("%d", ans[j]);
			}
		}
		printf("\n");
		ans.clear();
	}
	return 0;
}

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

int main(){
	int x[100];
	int y[100];
	int z[100];
	int n, i, j, hi;
	bool top;
	string a, b;
	
	cin >> n;
	for (i = 0; i < n; i++) {
		// initialize
		for(j=0; j<100; j++){
			x[j] = 0;
			y[j] = 0;
			z[j] = 0;
		}
		cin >> a;
		cin >> b;
		if(a.length() > 80 || b.length() > 80){
			cout << "overflow" << endl;
			continue;
		}
		for (j = 0; j < a.length(); j++){
			x[j] = atoi(a.substr(a.length()-1-j, 1).c_str());
		}
		for (j = 0; j < b.length(); j++){
			y[j] = atoi(b.substr(b.length()-1-j, 1).c_str());
		}
		
		hi = 0;
		for(j=0; j<80; j++){
			z[j] = x[j] + y[j] + hi;
			if(z[j] > 9){
				hi = 1;
				z[j] -= 10;
			}else{
				hi = 0;
			}
		}
		if(hi == 1){
			cout << "overflow" << endl;
		}else{
			top = false;
			for (j = 79; j > -1; j--) {
				if(z[j] == 0 && !top){
					continue;
				}else{
					top = true;
					cout << z[j];
				}
			}
			cout << endl;
		}
	}
	return 0;
}