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 <iomanip>
#include <cassert>
#include <string>
#include <cstring>
#include <algorithm>

using namespace std;

typedef long long ll;

const ll BASE = 10000;
const int LogB = 4;
const int MAX_DIGIT = 100;

struct Bigint {
	ll digit[MAX_DIGIT];
	int size;
	Bigint (int sz = 1, ll a = 0) : size(sz) {
		memset(digit, 0, sizeof(digit));
		digit[0] = a;
	}
};

const Bigint ZERO(1,0), ONE(1, 1);

bool operator < (Bigint x, Bigint y) {
	if (x.size != y.size) return x.size < y.size;
	for (int i=x.size-1; i>=0; i--) if (x.digit[i] != y.digit[i]) return x.digit[i] < y.digit[i];
	return false;
}

bool operator > (Bigint x, Bigint y) { return y < x; }
bool operator <= (Bigint x, Bigint y) { return !(y < x); }
bool operator >= (Bigint x, Bigint y) { return !(x < y); }
bool operator != (Bigint x, Bigint y) { return x < y || y < x; }
bool operator == (Bigint x, Bigint y) { return !(x < y) && !(y < x); }

Bigint normal(Bigint x) {
	ll c = 0;
	for (int i=0; i<x.size; i++) {
		while (x.digit[i] < 0)
			x.digit[i+1] -= 1, x.digit[i] += BASE;
		ll a = x.digit[i] + c;
		x.digit[i] = a % BASE;
		c = a / BASE;
	}
	for (; c>0; c /= BASE) x.digit[x.size++] = c % BASE;
	while (x.size > 1 && x.digit[x.size-1] == 0) x.size--;
	return x;
}

Bigint convert(ll a) { return normal(Bigint(1, a)); }

Bigint convert(const string& s) {
	Bigint x;
	int i = s.size() % LogB;
	if (i > 0) i -= LogB;
	for (; i<(int)s.size(); i += LogB) {
		ll a = 0;
		for (int j=0; j<LogB; j++) a = 10 * a + (i+j >= 0 ? s[i+j] - '0' : 0);
		x.digit[x.size++] = a;
	}
	reverse(x.digit, x.digit+x.size);
	return normal(x);
}

ostream &operator << (ostream& os, Bigint x) {
	os << x.digit[x.size-1];
	for (int i=x.size-2; i>=0; i--) os << setw(LogB) << setfill('0') << x.digit[i];
	return os;
}

istream &operator >> (istream& is, Bigint &x) {
	string s; is >> s;
	x = convert(s);
	return is;
}

Bigint operator + (Bigint x, Bigint y) {
	if (x.size < y.size) x.size = y.size;
	for (int i=0; i<y.size; i++) x.digit[i] += y.digit[i];
	return normal(x);
}

int main() {
	int n;
	cin >> n;
	string s1, s2;
	
	for (int i=0; i<n; i++) {
		cin >> s1;
		cin >> s2;
		Bigint x = convert(s1), y = convert(s2);
		Bigint z = x + y;
		if (z.size > 80) cout << "overflow" << endl;
		else cout << z << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#define N 10000
using namespace std;
void mpstr2num(int *num, char *str);
void mpadd(int* ret, int* a, int* b);
void mpnum2str(char* str, int* num);

void mpstr2num(int* num, char* str){
    char* ss;
    int* nn;
    while(*str == ' ' || *str == '\t') str++;
    while(*str == '0') str++;
    ss = str;
    while(*ss >= '0' && *ss <= '9') ss++;
    if(ss == str){
        *num = 0;
        return;
    }
    int x = 0;
    int k = 1;
    nn = num;
    do{
        x+=(*--ss - '0') * k;
        k *= 10;
        if(k == N || ss == str){
            *++nn = x;
            x = 0;
            k = 1;
        }
    }while(ss != str);

    *num = nn - num;
    return;
}

void mpadd(int* ret, int* a, int* b){
    int la = *a;
    int lb = *b;
    int lr = (la >= lb) ? la : lb;
    int* rr = ret;
    int x = 0;
    for(int i=1; i<= lr; i++){
        if(i <= la) x += *++a;
        if(i <= lb) x += *++b;
        if(x < N){
            *++rr = x;
            x = 0;
        }else{
            *++rr = x - N;
            x = 1;
        }
    }
    *++rr = x;
    *ret = lr + x;
    return;
}

void mpnum2str(char* str, int* num){
    if(*num == 0){
        *str++ = '0';
        *str = '\0';
        return;
    }
    char* ss = str - 1;
    int x;
    for(int i=*num; i>0; i--){
        x = *++num;
        for(int j=1; j<N; j*=10){
            *++ss = x%10 + '0';
            x /= 10;
        }
    }
    while(*ss == '0') ss--;

    *(ss+1) = '\0';
    while(str < ss){
        x = *str;
        *str++ = *ss;
        *ss-- = x;
    }

    return;
}

int main(){
    int n;
    cin >> n;
    for(int i=1; i<=n; i++){
        char a[100], b[100];
        int anum[25], bnum[25];
        int resultnum[50];
        char result[200];
        cin >> a >> b;
        mpstr2num(anum, a);
        mpstr2num(bnum, b);
        mpadd(resultnum, anum, bnum);
        mpnum2str(result, resultnum);
        cout << result << endl;
    }
    return 0;
}