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<algorithm>
#include<string>


#define MAX 1000


//99999999999999999999999999999999999999999999999999999999999999999999999999999999

using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

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

		in[0].resize(0);
		in[1].resize(0);

		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;


		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		for (i = 0; out[i]; i++){
			;
		}

		if (i > 80){
			cout << "overflow" << endl;
		}else{
			cout << out << endl;
		}
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <sstream>
#include <math.h>
using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int kk = 0; kk < n; ++kk)
	{
		int A[1000] = {},B[1000] = {},C[1000] = {},keta = 0;
		string in1,in2;
		cin >> in1 >> in2;
		if(in1.size() >= 80 || in2.size() >= 80)
		{
			cout << "overflow" << endl;
			continue;
		}
		for (int i = 0; i < in1.size(); ++i)
			A[in1.size()-i] = (int)(in1[i]-'0');
		for (int i = 0; i < in2.size(); ++i)
			B[in2.size()-i] = (int)(in2[i]-'0');
		bool flag = 1;
		for (int i = 1; i < 1000; ++i)
		{
			C[i] += A[i]+B[i];
			if(C[i] >= 10)
			{
				C[i+1] += C[i]/10;
				C[i] -= C[i]/10*10;
			}
			if(C[i] != 0)
				keta = i;
		}
		if(keta >= 80)
		{
			cout << "overflow" << endl;
			continue;
		}
		cout << "[" << keta << "]" << endl;
		for (int i = keta; i > 0; --i)
		{
			cout << C[i];
		}
		cout << endl;
	}
}

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

class largeInt{
public:
	largeInt() : digit( 0 ){
		initNum();
	}
	~largeInt(){}

	void set( const string& s ){
		numStr = s;
		initNum();
		digit = numStr.size();
		if ( digit > 80 ) return;
		StoI();
	}
	const char* getFigure(){ return numStr.c_str(); }
	const int getDigit(){ return digit; }

	void operator+=( const largeInt& a ){
		numStr = "";
		for ( int i = 0; i < 10; ++i ){
			num[ i ] += a.num[ i ];
			if ( num[ i ] / 100000000 && i != 9 ){
				num[ i ] -= 100000000;
				num[ i + 1 ] += 1;
			}
			int t = num[ i ];
			string s;
			for ( int j = ( i == 9 ? 8 : 7 ); j >= 0; --j ){
				int power = static_cast< int >( pow( 10.0, static_cast< double >( j ) ) );
				int c = t / power;
				t %= power;
				s += static_cast< char >( c + '0' );
			}
			numStr = s + numStr;
			if ( i == 9 ){
				string::iterator it = numStr.begin();
				while ( ( *it == '0' ) && ( it != numStr.end() - 1 ) ){
					it = numStr.erase( it );
				}
			}
		}
		digit = numStr.size();
	}

private:
	void initNum(){
		for ( int i = 0; i < 10; ++i ){
			num[ i ] = 0;
		}
	}
	void StoI(){
		bool f = false;
		for ( int i = 0; i < digit / 8; ++i ){
			for ( int j = 0; j < 8; ++j ){
				if ( i*8 + j >= digit ){
					f = true;
					break;
				}else{
					conv( i, j );
				}
			}
			if ( f ) break;
		}
		for ( int i = 0; i < digit % 8; ++i ){
			conv( digit / 8, i );
		}
	}
	void conv( int i, int j ){
		int figure = static_cast< int >( numStr[ ( digit - 1 ) - ( i*8 + j ) ] - '0' );
		int power = static_cast< int >( pow( 10.0, static_cast< double >( j ) ) );
		num[ i ] += figure * power;
	}

	unsigned num[10];
	int digit;
	string numStr;
};

int main(){
	int n;
	string s;

	cin >> n;

	largeInt* a = new largeInt[ n ];
	largeInt* b = new largeInt[ n ];

	for ( int i = 0; i < n; ++i ){
		cin >> s;
		a[ i ].set( s );
		cin >> s;
		b[ i ].set( s );
	}

	for ( int i = 0; i < n; ++i ){
		if ( a[ i ].getDigit() > 80 || b[ i ].getDigit() > 80 ){
			cout << "overflow" << endl;
			continue;
		}
		a[ i ] += b[ i ];
		cout << ( a[ i ].getDigit() > 80 ? "overflow" : a[ i ].getFigure() ) << endl;
	}

	delete[] a;
	delete[] b;

	return 0;
}