Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

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

int main() {
	int n;
	cin >> n;

	for (int i = 0; i < n; i++) {
		string num1, num2;
		cin >> num1 >> num2;
		if (num1.size() < num2.size()) swap(num1, num2);
		if (num1.size() > 80) {
			cout << "overflow\n";
			continue;
		}

		reverse(num1.begin(), num1.end());
		reverse(num2.begin(), num2.end());

		for (int j = 0; j < num2.size(); j++) {
			int p = (num1[j] - '0') + (num2[j] - '0');
			if (p >= 10) {
				num1[j] = ((num1[j] - '0') + (num2[j] - '0')) % 10 + '0';

				int k = j + 1;
				while (num1[k] == '9' && k < num1.size()) {
					num1[k++] = '0';
				}
				
				if (k == num1.size()) num1 = num1 + "1";
				else num1[k]  = num1[k] + 1;
			}
			else {
				num1[j] = num1[j] + (num2[j] - '0');
			}
		}
		if (num1.size() > 80) {
			cout << "overflow\n";
			continue;
		}
		reverse(num1.begin(), num1.end());
		cout << num1 << endl;
	}
}

C++: #include<iostream>
#include<cstdio>
#include<vector>
#include<stack>
using namespace std;

int main()
{
  int n;
  cin >> n;
  getchar();
  while( n --){
    int i;
    int A[500] = {} , B[500] = {};
    stack<int> C;
    char a, b;
    int end;
    i = 100;
    while(1) {
      a = getchar();
      if(a == '\n') break;
      A[i] = a - '0';
      i--;
      if(i < 20) break;

    }
    int j = 100;
    while(1) {
      b = getchar();
      if(b == '\n') break;
      B[j] = b - '0';
      j--;
      if(j < 20)  break; 

    }
    if( i < 20  || j < 20 ) {  cout << "overflow" <<endl; continue; }

    if(i > j)
      end = 101 - j;
    else
      end = 101 - i;

    A[i++] += B[j++];
  
    for(int k = 0; k <= end ; i++, j++, k++) {
      A[i] += B[j];
      A[i] += A[i - 1] / 10;
      A[i - 1] %= 10;
      C.push(A[i - 1]);
    }
    if(C.top() == 0 && C.size() != 1) {
      C.pop();
    }
    end = C.size() - 1;
    if(end > 80){
      cout << "overflow" << endl;
      continue;
    }
    // cout << C.size() << endl;
    for(i = 0; i < end; i++) {
      cout << C.top();
      C.pop();
    }
  
    cout << endl;
  
  }
}

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<iostream>
#include<string>
#include<cstdio>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<cmath>
#include<utility>
#include<set>
#include<complex>
#define vi vector<int>
#define vvi vector<vector<int> >
#define ll long long int
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vb vector<bool>
#define vc vector<char>
#define vs vector<string>
#define ld long double
#define INF 1e9
#define EPS 0.0000000001
#define rep(i,n) for(int i=0;i<n;i++)
#define CC puts("-------ok--------");
#define all(in) in.begin(), in.end()
#define bv vector<bool>
using namespace std;
typedef pair<int, int>PA;
using namespace std;
#define MAX  999999
int main (){
    int n; cin >> n;
    rep(k,n){
        string s; string s2;
        cin >> s >> s2;
        vi v(100,0);
        if(s.length()>80 || s2.length() > 80){
            cout << "overflow" << endl;
        }else{
            vi v(100,0); vi v2(100,0);
            vi ans(100,0);
            if(s.length()<s2.length()) swap(s,s2);
            rep(i,s.length())
                v[i] = (int)s[i]-48;
            rep(i,s2.length())
                v2[i] = (int)s2[i]-48;
            int num = (int)max(s2.length(),s.length());
            reverse(v.begin(), v.begin()+s.length());
            reverse(v2.begin(),v2.begin()+ s2.length());
            int ansize = 0;
            rep(i,num){
                ansize++;
                ans[i] += v[i]+v2[i];
                if(ans[i] >= 10){
                    ans[i]-=10;
                    ans[i+1]+=1;
                    if(i == num-1)
                        ansize++;
                }
            }
            if(ansize >80)
                cout <<"overflow" << endl;
            else{
            reverse(ans.begin(), ans.begin()+ansize);
            rep(i,ansize)
            cout << ans[i];
            cout << endl;
            }
        }
        
        }
        return 0;
}

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

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; ++i)
	{
		int A[150] = {},B[150] = {},C[150] = {};
		string inA,inB;
		cin >> inA >> inB;
		for (int j = 0; j < inA.size(); ++j)
			A[inA.size()-j] = (int)(inA[j]-'0');
		for (int j = 0; j < inB.size(); ++j)
			B[inB.size()-j] = (int)(inB[j]-'0');
		int keta = 0;
		for (int j = 1; j < 150; ++j)
		{
			C[j] += A[j] + B[j];
			if(C[j] >= 10)
			{
				C[j+1] += C[j]/10;
				C[j] = C[j]%10;
			}
			if(C[j] != 0)
				keta = j;
		}
		cout << keta << endl;
		if(keta > 80)
			cout << "overflow" << endl;
		else
		{
			bool haji = 0;
			for (int j = 149; j > 0; --j)
			{
				if(C[j] != 0)
					haji = 1;
				if(haji)
				{
					cout << C[j];
				}
			}
			cout << endl;
		}
	}
}