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<cstdio>
#include<string>
#include<algorithm>
#include<math.h>
using namespace std;
bool isover(int);
bool isover(int,int);
int main() {
	int n; string ans; bool is_str_use;
	string str, str1; int nexta; int memo, age;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		age = 0; ans = ""; is_str_use = false;
		cin >> str >> str1;
		if (isover(str.size(), str1.size())) {
			cout << "overflow" << endl;
			continue;
		}
		for (int i = 0; i < str.size() / 2; ++i) {
			swap(str[i], str[str.size() - 1 - i]);

		}
		for (int i = 0; i < str1.size() / 2; ++i) {
			swap(str1[i], str1[str1.size() - 1 - i]);
		}
		if (str.size() < str1.size()) {
			is_str_use = true;
		}
		for (int i = 0; i < min(str.size(), str1.size()); ++i) {
			memo = (str[i] - '0') + (str1[i] - '0') + age;
			if (memo > 9) {
				age = memo / 10;
				memo -= (memo / 10) * 10;
			}
			else {
				age = 0;
			}
			ans += memo + '0';
		}
		if (str.size() == str1.size()&&age!=0) {
			ans += age + '0';
			age = 0;
		}
		if (isover(ans.size())) {
			cout << "overflow" << endl;
			continue;
		}
		if (is_str_use) {
			swap(str, str1);

		}
		for (int i = min(str.size(), str1.size()); i < str.size(); ++i) {
			memo = (str[i] - '0') + age;
			if (memo > 9) {
				age = memo / 10;
				memo -= (memo / 10) * 10;
			}
			else {
				age = 0;
			}
			ans += memo + '0';
		}
		if (age != 0)ans += age+'0';
		if (isover(ans.size())) {
			cout << "overflow" << endl;
			continue;
		}
		for (int i = 0; i < ans.size() / 2; ++i) {
			swap(ans[i], ans[ans.size() - 1 - i]);
		}
		cout << ans << endl;
	}
	return 0;
}
bool isover(int a) {
	if (a > 80) {
		return true;
	}
	return false;
}
bool isover(int a, int b) {
	if (a > 80 || b > 80) {
		return true;
	}
	return false;
}

C++: #include <stdio.h>

void rev_str(char str[128]) {
	int length = 0;
	char str2[128];
	int temp;

	int i;

	for(i = 0; i < 128; i++)
		str2[i] = '\0';

	while(str[length])
		length++;

	temp = length;
	i = 0;
	while(temp) {
		str2[temp - 1] = str[i];
		i++;
		temp--;
	}

	for(i = 0; i < length; i++)
		str[i] = str2[i];
}

int exor(int a, int b) {
	if(a && b)
		return 0;
	else if(a || b)
		return 1;
	else
		return 0;
}

int main() {
	char input_a[128];
	char input_b[128];
	char ans[286][128];
	char kuriagari;
	int temp;
	int n;

	int i, j;

	scanf("%d", &n);

	for(j = 0; j < n; j++) {
		for(i = 0; i < 128; i++) {
			input_a[i] = '\0';
			input_b[i] = '\0';
			ans[j][i] = '\0';
		}

		kuriagari = 0;

		scanf("%s", input_a);
		scanf("%s", input_b);

		rev_str(input_a);
		rev_str(input_b);

		i = 0;
		while(input_a[i] && input_b[i]) {
			temp = input_a[i] + input_b[i] - 2 * '0' + kuriagari;
			ans[j][i] = temp % 10 + '0';
			kuriagari = temp / 10;
			i++;
		}

		if(kuriagari && !input_a[i] && !input_b[i]) {
			ans[j][i] = kuriagari + '0';
		}

		else {
			while (kuriagari && exor((int)input_a[i], (int) input_b[i])) {
				temp = input_a[i] + input_b[i] - '0' + kuriagari;
				ans[j][i] = (char)(temp % 10 + '0');
				kuriagari = temp / 10;
				i++;
			}

			if (kuriagari)
				ans[j][i] = kuriagari + '0';
		}

		while(input_b[i]) {
			ans[j][i] = input_b[i];
			i++;
		}

		while(input_a[i]) {
			ans[j][i] = input_a[i];
			i++;
		}

		rev_str(ans[j]);

	}

	for(j = 0; j < n; j++) {
		if(ans[j][81])
			printf("overflow");
		else
			printf("%s", ans[j]);

		putchar('\n');
	}

	return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
  string s,n,final;
  int o,c=0;
  cin>>s>>n;
  reverse(s.begin(),s.end());
  reverse(n.begin(),n.end());
  for(int i = 0; i < s.size() || i<n.size() || c;i++){
    o = (s.size() > i ? s[i]-'0' : 0) +(n.size() > i ? n[i]-'0' : 0) + c;

    //   o = (s[i]-'0')+(n[i]-'0')+c;
    c = o / 10;
    o %= 10;
    final.append(1,o+'0');
  }
  reverse(final.begin(),final.end());
  if(n.size()>=80 || s.size()>=80 || final.size()>=80) cout<<"overflow"<<endl;
  else cout<<final<<endl;
}