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<bits/stdc++.h>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<algorithm>
using namespace std;


int main(){
    int t;
    scanf("%d",&t);
    while(t--){
        char c1[80],c2[80];
        scanf("%s",&c1);
        scanf("%s",&c2);
        string s1,s2;
        s1=c1, s2=c2;
        if(s1=="0" && s2=="0"){
            printf("0\n");
            continue;
        }
        reverse(s1.begin(),s1.end());
        reverse(s2.begin(),s2.end());
        int n=max(s1.size(),s2.size())+1;
        for(int i=s1.size();i<n;i++){
            s1+='0';
        }
        for(int i=s2.size();i<n;i++){
            s2+='0';
        }
        string res;
        int b=0;
        for(int i=0;i<max(s1.size(),s2.size());i++){
            int u=s1[i]-'0'+s2[i]-'0'+b;
            b=(u>=10); u%=10;
            res+=u+'0';
        }

        reverse(res.begin(),res.end());
        string res2;
        int j=0;
        while(res[j]=='0'){
            j++;
        }
        for(;j<res.size();j++){
            res2+=res[j];
        }

        if(res2.size()>80){
            printf("overflow\n");
        }else{
            printf("%s\n",res2.c_str());
        }
    }
    return 0;
}

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

using namespace std;
string a,b,sum;
void check(int z,int x);
int A,B;
int main(void){

    int n;
    cin>>n;
    while(n--){
        a="",b="",sum="";

        cin>>a>>b;
        //cout<<"a="<<a<<"b="<<b<<"sum="<<sum<<endl;
        if(a.length()>80||b.length()>80){
            sum="overflow";
            goto end;
        }
        A=a.length(),B=b.length();
        //cout<<A<<B<<endl;
        reverse(a.begin(),a.end());
        reverse(b.begin(),b.end());
        if(A<B)swap(A,B),swap(a,b);
        for(int i=0;i<=A-B;i++){
            b+="0";
        }
        check(0,0);
        //cout<<sum<<endl;
        if(sum.length()>80){
            sum="overflow";
            goto end;
        }
        reverse(sum.begin(),sum.end());
        end:;
        cout<<sum<<endl;
    }
    return 0;
}

void check(int z,int x){
    int i;


    if(a[z]==0)a[z]='0';
    if(b[z]==0)b[z]='0';
    //cout<<a[z]<<"."<<b[z]<<"."<<sum<<endl;
    //cout<<"--"<<z<<endl;
    if(z==81||(z>A&&z>B&&x==1)||(z>=A&&z>=B&&x==0)){
            x=0;
            return;
    }
    i=(int)a[z]-'0' + (int)b[z]-'0' + x,x=0;
    //cout<<"*"<<i<<endl;
    //cout<<"-"<<z<<endl;
    if(i>=10)x=1,i-=10;
    sum+=i+'0';
    //cout<<"+"<<sum<<endl;

    check(z+1,x);

}

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

int main() {
	int n;
	cin >> n;
	for (int x = 0;x < n;x++) {
		string a, b;
		cin >> a >> b;
		int za = a.size();
		int zb = b.size();
		vector<int>v;

		for (int i = 0;i < max(za, zb);i++) {
			if (i < min(za, zb)) {
				int c = za - i - 1;
				int d = zb - i - 1;
				v.push_back((a[za - i - 1] - '0') + (b[zb - i - 1] - '0'));
			}
			else {
				if (za < zb)v.push_back(b[zb - i - 1] - '0');
				else v.push_back((a[za - i - 1]) - '0');
			}
		}
		int z = v.size();
		for (int i = 0;i < v.size();i++) {
			if (i != z- 1 && v[i] >= 10) {
				v[i] -= 10;
				v[i + 1]++;
			}
		}
		if (v.size() >= 81)cout << "overflow";
		else 	for (int i =z - 1;i > -1;i--)cout << v[i];
		cout << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>

int main(void)
{
  int N;
  scanf("%d", &N);

  int i,j,k;
  for(i=0; i<N; ++i){

    int digits[2][80];
    for(j=0; j<2; ++j){
      for(k=0; k<80; ++k){
        digits[j][k] = 0;
      }
    }

    int overflow = 0; 

    for(j=0; j<2; ++j){
      char digit[256];
      scanf("%256s", digit);
      getchar();
      
      if(overflow || strlen(digit)>80){
        overflow = 1;
      }else{
        for(k=0; k<strlen(digit); ++k){
          digits[j][k] = digit[strlen(digit)-1-k]-'0';
        }
      }
    }

    if(!overflow){
      int sum[81];
      for(j=0; j<81; ++j){
        sum[j] = 0;
      }

      for(j=0; j<80; ++j){
        for(k=0; k<2; ++k){
          sum[j] += digits[k][j];
        }
        sum[j+1] += sum[j]/10;
        sum[j] %= 10;
      }

      if(sum[80]>0){
        overflow = 1;
      }else{
        int available = 0;
        for(j=80; j>=0; --j){
          if(available || sum[j]!=0 || j==0){
            available = 1;
            printf("%d", sum[j]);
          }
        }
        printf("\n");
      }
    }

    if(overflow){
      printf("overflow\n");
    }
  }

  return 0;
}