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>
using namespace std;

string IntToString(int number) {
    stringstream ss;
    ss << number;
    return ss.str();
}

int main(){
    int n;
    cin >> n;
    for(int p=0; p < n; p++){
        string ans;
        string s, t;
        cin >> s >> t;
        if(s.size() > t.size()) swap(s, t);


        int q = t.size()-s.size();
        for(int i=0; i < q; i++) s = "0" + s;

        int up = 0;
        for(int i=s.size()-1; 0 <= i; i--){
            int d = atoi(s.substr(i, 1).c_str());
            int f = atoi(t.substr(i, 1).c_str());
            ans = IntToString((d+f+up)%10) + ans;

            if(0 <= d+f+up-10) up = 1;
            else up = 0;
        }
        if(up == 1) ans = IntToString(1) + ans;

        if(80 < ans.size()) cout << "overflow" << endl;
        else cout << ans << endl;
     }
}

C++: #include <iostream>
#include <stdio.h>
#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <algorithm>
#include <set>
#include <math.h>
#include <utility>
#include <stack>
using namespace std;
typedef pair<int,int> P;
const int INF = ~(1<<31) / 2;

int main(){
    int k;
    cin >> k;
    for(int j=0;j<k;j++){
        int n[80];
        string a,b;
        cin >> a >> b;
        int n_a = a.size(),n_b = b.size();
        if(n_a>80||n_b>80){cout << "overflow" << endl;continue;}

        fill(n,n+80,0);
        bool is_over = false;
        for(int i=0;i<max(n_a,n_b);i++){
            int ai,bi;
            if (i<n_a) ai = a[n_a-i-1]-'0';
            else ai = 0;
            if (i<n_b) bi = b[n_b-i-1]-'0';
            else bi = 0;
            n[i] += ai + bi;
            if (n[i]>=10){
                if(i+1 == 80){
                    is_over = true;
                    break;
                }else{
                    n[i+1] += n[i]/10;
                    n[i] = n[i]%10;
                }
            }
        }
        if (is_over){
            cout << "overflow" << endl;
        }else{
            bool started = false;
            for(int i=79;i>=0;i--){
                if(!started && n[i]>0){
                    started = true;
                }
                if(started || (!started&&i==0)){
                    cout << n[i];
                }
            }
            cout << endl;
        }
    }
    return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
typedef vector <int> vi;
typedef vector <string> vs;

string right(string &text, int n){
	int length = text.size();
	string tmp;
	tmp = text.substr(length-n, n);
	text = text.substr(0, length-n);
	return tmp;
}

long str2lng(string text){
	stringstream ss;
	ss<<text;
	long tmp;
	ss>>tmp;
	return tmp;
}

string lng2str(long n){
	stringstream ss;
	ss<<n;
	string text;
	ss>>text;
	return text;
}

vi longstr2num(string text){
	vi buf;
	while (!text.empty()){
		if (text.size()>=8)
			buf.push_back(str2lng(right(text, 8)));
		else{
			buf.push_back(str2lng(text));
			text="";
		}
	}
	return buf;
}

int main(void){
	int n;
	cin>>n;
	for (int i=0; i<n; i++){
		string s1, s2;
		cin>>s1>>s2;
		vi tmp, tmp1 = longstr2num(s1), tmp2 = longstr2num(s2);
		int size1 = tmp1.size(), size2 = tmp2.size(), size=max(size1, size2);
		long s, c = 0;
		for (int j=0; j<size; j++){
			s = c;
			if (j<size1) s += tmp1[j];
			if (j<size2) s += tmp2[j];
			
			tmp.push_back(s%100000000);
			c = s / 100000000;
		}
		if (c != 0)	tmp.push_back(c);

		stringstream buf;
		for (int j=tmp.size()-1; j>=0; j--)
			if (j != tmp.size()-1)
				buf<<setfill('0')<<setw(8)<<tmp[j];
			else
				buf<<tmp[j];
		string str;
		buf>>str;
		if (str.size()>80) cout<<"overflow"<<endl;
		else cout<<str<<endl;
	}
	return 0;
}

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

using namespace std;

int main(int argc, char* argv[])
{
	int i;
	int num, pos;
	int len1, len2;
	char temp;
	char f1[101] = { 0 }, f2[101] = { 0 };
	int  up[100] = { 0 };
	int  sum[101] = { 0 };

	cin >> num;
	while (num--) {
		cin >> f1;
		cin >> f2;

		len1 = strlen(f1);
		len2 = strlen(f2);
		if (len1 > 80 || len2 > 80) {
			cout << "overflow" << endl;
			continue;
		}

		for (int i = 0; i < len1 / 2; i++) {
			temp = f1[i];
			f1[i] = f1[len1 - 1 - i];
			f1[len1 - 1 - i] = temp;
		}
		for (int i = 0; i < len2 / 2; i++) {
			temp = f2[i];
			f2[i] = f2[len2 - 1 - i];
			f2[len2 - 1 - i] = temp;
		}

		for (i = 0; i < len1; i++) f1[i] = f1[i] - 0x30;
		for (i = 0; i < len2; i++) f2[i] = f2[i] - 0x30;

		for (int i = 0; i < 100; i++) {
			sum[i] += (f1[i] + f2[i]);
			if (sum[i] >= 10) {
				sum[i] -= 10;
				sum[i + 1]++;
			}
		}

		for (pos = 100; sum[pos] == 0; pos--) {}
		if (pos >= 80) {
			cout << "overflow";
			break;
		}
		for (int i = pos; i >= 0; i--) {
			cout << sum[i];
		}
		cout << endl;
	}
}