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<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
int main() {
	int n;
	string a,b;
	cin >> n;
	while(n--) {
		cin >> a >> b;
		string ret = "";
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		int a_len = a.size(),b_len = b.size();
		int num = max(a_len,b_len),carry = 0,v;
		char tmp[1];
		for(int i = 0; i < num; i++) {
			if(a_len <= i || b_len <= i) v = carry + (b_len <= i ? a[i] : b[i]) - '0';
			else v = carry + a[i] + b[i] - '0' * 2;
			carry = v / 10;
			sprintf(tmp,"%d",v % 10);
			ret += tmp;
		}
		sprintf(tmp,"%d",carry);
		if(carry != 0) ret += tmp;
		reverse(ret.begin(),ret.end());
		cout << (ret.size() > 80 ? "overflow" : ret) << endl;
	}
	return 0;
}

C++: #include <cassert>
#include <iostream>
#include <string>
#include <vector>

typedef std::vector<char> TLongInt;
TLongInt Decode(const std::string& Code)
{
    std::vector<char> Value(100, 0);
    for(int i = 0; i < Code.size(); ++i){
        Value[i] = Code[Code.size() -1 -i] - '0';
    }
    return Value;
}
TLongInt Add(const TLongInt& A, const TLongInt& B)
{
    assert(A.size() == B.size());
    TLongInt C(A.size(), 0);
    int kuriagari = 0;
    for(int i = 0; i < A.size(); ++i){
        int V = A[i] + B[i] + kuriagari;
        C[i] = V % 10;
        kuriagari = V / 10;
    }
    return C;
}

void Print(const TLongInt& Value, std::ostream& out)
{
    bool Left = true;
    for(int i = Value.size() - 1; i >= 0; --i){
        if(Value[i] == 0 && Left) continue;
        if(i >= 80){
            out << "overflow" << std::endl;
            return;
        }
        Left = false;
        out << int(Value[i]);
    }
    out << std::endl;
}

int main()
{
    int N;
    std::cin >> N;
    for(int i = 0; i < N; ++i){
        std::string first, second;
        std::cin >> first;
        std::cin >> second;
        Print(Add(Decode(first), Decode(second)), std::cout);
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <deque>
class BigInt
{
  std::deque<char> m_digits;
public:
  BigInt(std::string str) {
    for(auto it = str.begin(); it != str.end(); ++it) {
      m_digits.push_back(*it - '0');
    }
  }

  BigInt(int num) {
    std::ostringstream oss;
    oss << num;
    BigInt(oss.str());
  }

  std::string str() const {
    std::string str;
    for (auto it = m_digits.begin(); it != m_digits.end(); ++it) {
      str += *it + '0';
    }
    return str;
  }

  const BigInt& operator +=(const BigInt& rhs) {
    std::deque<char> new_digits;
    auto lhs_rit = m_digits.rbegin();
    auto rhs_rit = rhs.m_digits.rbegin();

    char inc = 0;
    while (lhs_rit != m_digits.rend() &&
           rhs_rit != rhs.m_digits.rend()) {
      char temp = *lhs_rit + *rhs_rit;
      temp += inc;
      if (temp >= 10) {
        inc = temp / 10;
        temp -= inc * 10;
      }
      new_digits.push_front(temp);
      ++lhs_rit;
      ++rhs_rit;
    }

    if (lhs_rit != m_digits.rend()) {
      while (lhs_rit != m_digits.rend()) {
        char temp = *lhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++lhs_rit;
      }
    }
    else if (rhs_rit != rhs.m_digits.rend()) {
      while (rhs_rit != rhs.m_digits.rend()) {
        char temp = *rhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++rhs_rit;
      }
    }
    if (inc > 0) new_digits.push_front(inc);
    m_digits = new_digits;
  }
};

BigInt operator +(const BigInt& lhs, const BigInt& rhs)
{
  BigInt result(lhs);
  result += rhs;
  return result;
}

int main()
{
  int n;
  std::cin >> n;
  while (n--) {
    std::string num;
    std::cin >> num;
    BigInt num1(num);
    std::cin >> num;
    BigInt num2(num);
    
    BigInt num3 = num1 + num2;
    std::cout << num3.str() << std::endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<algorithm>
#include<cstring>
#include<string>
using namespace std;
int main(void)
{
	int y;
	scanf("%d",&y);
	for(int z = 0;z < y;z++)
	{
		char suji1[100000],suji2[100000];
		char ans[100000] = {0};

		for(int i = 0;i<82;i++)
		{
			suji1[i] = '0';
			suji2[i] ='0';
		}

		int overflow = 0;

		scanf("%s",suji1);
		scanf("%s",suji2);
		int suji1s = strlen(suji1);
		int suji2s = strlen(suji2);

		int anss;
		if(suji1s>suji2s)  anss = suji1s;
		else anss = suji2s;

		suji1[suji1s] = '0';
		suji2[suji2s] = '0';

		reverse(suji1,suji1+suji1s);
		reverse(suji2,suji2+suji2s);

		for(int i = 0;i < 80;i++)
		{
			ans[i] = suji1[i]+suji2[i] - '0';
			if(ans[i] >= ':')
			{
				ans[i]  = ans[i] - 10;
				suji2[i+1] = suji2[i+1]+1;
			}
		}
		
		if(ans[anss]!= '0') anss++;
		if(anss >=80) overflow = 1;
		reverse(ans,ans+anss);
		ans[anss] = 0;
		if(overflow >= 1)
		{
			printf("overflow\n");
		}
		else
		{
			printf("%s\n",ans);
		}
		
	}
	return 0;
}