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<string>
#include<algorithm>
using namespace std;

int main()
{
	string s1, s2, sum;
	int n;
	cin >> n;
	while (n--) {
		cin >> s1 >> s2;
		int len = max(s1.size(), s2.size());
		for (int i = 0; i < len; i++) {
			sum += "0";
		}
		// ?????????????????????????????????????????????'0'??§??°????????????
		if (s1.size() < s2.size()) {
			// s1????????????????????§s2.size()-s1.size()?????????'0'?????°????????????
			// ??????s1??????????????????
			string r_s1 = "";
			for (int i = s1.size() - 1; i >= 0; i--) {
				r_s1 += s1[i];
			}
			// r_s1???s2.size()-s1.size()?????????'0'???????????°??????
			for (int i = 0; i < s2.size() - s1.size(); i++) {
				r_s1 += "0";
			}
			// r_s1????????????????????????
			s1 = "";
			for (int i = r_s1.size() - 1; i >= 0; i--) {
				s1 += r_s1[i];
			}
		}
		else {
			// ??????s2??????????????????
			string r_s2 = "";
			for (int i = s2.size() - 1; i >= 0; i--) {
				r_s2 += s2[i];
			}
			// s1.size()-s2.size()?????????'0'??§????????°??????
			for (int i = 0; i < s1.size() - s2.size(); i++) {
				r_s2 += "0";
			}
			// r_s2??????????????????
			s2 = "";
			for (int i = r_s2.size() - 1; i >= 0; i--) {
				s2 += r_s2[i];
			}
		}
		
		int carry = 0;
		char s;
		for (int i = 0; i < len; i++) {
			if (carry == 0) {
				s = s1[s1.size() - 1 - i] - '0' + s2[s2.size() - 1 - i] - '0' + '0';
				if (s - '0' >= 10) {
					carry = 1;
					sum[len - 1 - i] = int((s - '0') % 10) + '0';
				}
				else {
					sum[len - 1 - i] = s;
				}
			}
			else {// carry over
				s = s1[s1.size() - 1 - i] - '0' + s2[s2.size() - 1 - i] - '0' + '0' + 1;
				if (s - '0' >= 10) {
					carry = 1;
					sum[len - 1 - i] = int((s - '0') % 10) + '0';
				}
				else {
					carry = 0;
					sum[len - 1 - i] = s;
				}
			}
		}

		cout << (carry == 1 ? "1" : "");
		for (int i = 0; i < len; i++) {
			cout << sum[i] - '0';
		}
		cout << endl;
	}
    return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <limits.h>
#include <string>
#include <stdlib.h>
#include <iostream>
using namespace std;

int main() {
	int lop;
	for (cin >> lop; lop > 0; lop--) {
		string a; cin >> a;
		string b; cin >> b;
		string c, d;
		int dif = a.size() - b.size();
		bool kuri = 0;
		for (int lop = 0; lop < abs(dif); lop++) {
			d += '0';
		}
		if (dif > 0)
			b = d + b;
		if (dif < 0)
			a = d + a;
		for (int i = 0; i < a.size(); i++) {
			c += '\0';
		}
		for (int i = a.size()-1; i >= 0; i--) {
			c[i] = a[i] + b[i]+kuri-'0';
			//cout << "c["<<i<<"] = "<<c[i] << endl;
			kuri = 0;
			if (c[i] > '9') {
				c[i] -= 10;
				kuri = 1;
			}
			if (i == 0 && kuri == 1)
				c = '1' + c;
		}
		if (c.size() > 80)
			cout << "overflow\n";
		else
			cout << c << endl;
	}
	return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define vi vector<int>
#define pb push_back
#define INF 999999999
//#define INF (1LL<<59)

string addString(string a,string b ){
	if(a.size()<b.size())swap(a,b);         //len(a)>len(b)???
	b = string(a.size()-b.size(),'0')+b;    //b????????????a??¨??????????????????????????????0??§?????????
	
	string ans = string(a.size()+1,'0');    //a?????????+1 ?????????0??§???????????????
	for(int i=(int)a.size()-1;i>=0;i--){    //?¶??????? & ??°????????????
		ans[i+1] += (a[i]-'0') + (b[i]-'0');
		if(ans[i+1]>='0'+10){
			ans[i] += 1;
			ans[i+1] -= 10;
		}
	}
	string ret = "";                        //??????????????°???0????¶????
	rep(i,ans.size()){
		if(ans[i]!='0'||i==ans.size()-1){
			ret = ans.substr(i,ans.size()-i);
			break;
		}
	}
	return ret;
}


int main(){
	int n;
	cin>>n;
	rep(i,n){
		string a,b;
		cin>>a>>b;
		cout<<addString(a,b)<<endl;
	}
}

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

int main() {
	int n;cin>>n;
	string a,b;for(int j=0;j<n;j++){
	cin>>a>>b;
	int d=0;
	if(a.length()<b.length())
	{
		int aa=a.length();
		for(int i=0;i<b.length()-aa;i++)a='0'+a;
	}
	else
	{
		int bb=b.length();
		for(int i=0;i<a.length()-bb;i++)b='0'+b;
	}
	string ans="";
	for(int i=a.length()-1;i>=0;i--)
	{
		int aa=a[i]-'0';
		int bb=b[i]-'0';
		aa+=bb+d;d=0;
		if(aa>=10)
		{
			aa-=10;
			d++;
		}
		ans=char(aa+'0')+ans;
	}
	if(d)ans=char(d+'0')+ans;
	while(ans[0]=='0')
	{
		ans=ans.substr(1,ans.length()-1);
	}
	if(ans.length()>=80)cout<<"overflow"<<endl;
	else cout<<ans<<endl;}
	return 0;
}