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[1000],c2[1000];
        scanf("%s",&c1);
        scanf("%s",&c2);
        string s1,s2;
        s1=c1, s2=c2;
        if(s1=="0" && s2=="0"){
            printf("0\n");
            continue;
        }
        //?????¢?????????0????????§?????°???????????????
        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;

        //b?????°?????????????????¨??????????????¨
        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;

        //????????¨?????´???0??????????????????0??\????????°??????????????¨?????????????????¨???????????????????¶????
        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<iostream>
#include<string>
#include<algorithm>
using namespace std;

string strplus(string x,string y){
  if(x.size()>80||y.size()>80)return "overflow";
  string ans;
  int xa[80],ya[80],za[80];
  for(int i=0;i<80;i++)xa[i]=ya[i]=za[i]=0;
  for(int i=0;i<(int)x.size();i++)xa[i]=x[x.size()-1-i]-'0';
  for(int i=0;i<(int)y.size();i++)ya[i]=y[y.size()-1-i]-'0';
  for(int i=0;i<80;i++){
    za[i]+=xa[i]+ya[i];
    if(za[i]>=10){
      if(i==79)return "overflow";
      else za[i+1]++;
    }
    za[i]%=10;
  }
  int flg=0;
  for(int i=79;i>=0;i--){
    if(flg==0&&za[i]==0)continue;
    else{
      flg=1;
      ans.push_back(za[i]+'0');
    }
  }
  return ans;
}
int main(){
  int n;
  string a,b;
  cin>>n;
  while(n--){
    cin>>a>>b;
    cout<<strplus(a,b)<<endl;
  }
  return 0;
}

C++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
using namespace std;
int main(void)
{
	char num1[1002],num2[1002];
	int a1[1002],a2[1002],ans[1002];
	int i,j,n,k,l1,l2,kuriagari=0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		kuriagari=0;
		for(j=0;j<1002;j++)
		{
			num1[j]='\0';
			num2[j]='\0';
			a1[j] = 0;
			a2[j] = 0;
			ans[j] = 0;
		}
		scanf("%s",num1);
		//getchar();
		scanf("%s",num2);
		//getchar();
		l1 = strlen(num1);
		//cout << l1 << endl;
		l2 = strlen(num2);
		//printf("%s %s\n",num1,num2);
		for(j=0;j<l1;j++)
		{
			a1[j] = num1[l1-1-j] - '0';
		}
		for(j=0;j<l2;j++)
		{
			a2[j] = num2[l2-1-j] - '0';
		}
		for(j=0;j<1002;j++)
		{
			ans[j] = (a1[j] + a2[j] + kuriagari) % 10;
			if((a1[j] + a2[j] + kuriagari)>=10)
			{
				kuriagari = 1;
			}
			else
			{
				kuriagari = 0;
			}
		}
		bool p=false;
		/*for(int i=0;i<1002;i++)
		{
			cout << ans[i];
		}
		cout << endl;*/
		for(j=1001;j>=80;j--)
		{
			if(ans[j]!=0)
			{
				p=true;
				break;
			}
		}
		if(p)
		{
			printf("overflow\n");
		}
		else
		{
			for(j=79;j>=0;j--)
			{
				if(ans[j] != 0)
				{
					k = j;
					break;
				}
			}
			for(j=k;j>=0;j--)
			{
				printf("%d",ans[j]);
			}
			printf("\n");
		}
	}
	return 0;
}