Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	sc.Scan()
	dataset, _ := strconv.Atoi(sc.Text())
	var firstdata *big.Int

	for i := 0; i < dataset*2; i++ {
		n := new(big.Int)
		sc.Scan()
		data, _ := n.SetString(sc.Text(), 10)

		if i%2 == 0 {
			firstdata = data
			continue
		}

		result := new(big.Int)
		if result := result.Add(data, firstdata); len(result.String()) > 80 {
			fmt.Println("overflow")
			continue
		} else {
			fmt.Println(result)
		}
	}
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
const int INF=10000;
char a[1001],b[1001],c[1001];
void rev(int pnt,char *cp){
    for(int i=0;i<pnt/2;i++){
        char work;
        work=*(cp+i);
        *(cp+i)=*(cp+pnt-i-1);
        *(cp+pnt-i-1)=work;
    }
}
int main(){
    int n;
    cin>>n;
    while(n--){
        for(int i=0;i<1000;i++)a[i]=b[i]=c[i]=0;
        cin>>a>>b;

        int apnt,bpnt;
        for(int i=0;i<1000;i++)if(a[i]==0){apnt=i;break;}
        for(int i=0;i<1000;i++)if(b[i]==0){bpnt=i;break;}

        if(apnt>80 || bpnt>80){cout<<"overflow"<<endl;continue;}

        rev(apnt,a);
        rev(bpnt,b);
        int d[100];
        for(int i=0;i<100;i++)d[i]=0;
        int big=max(apnt,bpnt);
        for(int i=0;i<big;i++){
            if(i<apnt)d[i]+=a[i]-'0';
            if(i<bpnt)d[i]+=b[i]-'0';
        }
        for(int i=0;i<big;i++){
            if(d[i]>=10){
                d[i]%=10;
                d[i+1]++;
            }
        }
        if(d[big]==1)big++;
        if(big>80){
            cout<<"overflow"<<endl;
            continue;
        }
        for(int i=0;i<big;i++){
            c[i]=d[i]+'0';
        }
        rev(big,c);
        cout<<c<<endl;
    }
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<string>
#include<vector>
#include<algorithm>
#include<cstdlib>
using namespace std;
typedef long long int ll;

int main()
{
	int n;
	cin >> n;
	for(int i=0;i<n;i++){
		int a[80];
		string sa1,sa2,s1="",s2="";
		for(int i=0;i<80;i++)a[i]=0;
		cin >> sa1 >> sa2;
		if(sa1.length()>80||sa2.length()>80){
			printf("overflow\n");
			continue;
		}
		for(int j=0;j<80-sa1.length();j++){
			s1+='0';
		}
		s1+=sa1;
		for(int j=0;j<80-sa2.length();j++){
			s2+='0'; 
		}
		s2+=sa2;
		for(int i=0;i<80;i++){
			a[i]+=(s1[i]-'0')+(s2[i]-'0');
		}
		for(int i=79;i>=1;i--){
			a[i-1]+=a[i]/10;
			a[i]%=10;
		}
		if(a[0]>=10){
			printf("overflow\n");
			continue;
		}
		int x=0;
		while(a[x]==0)x++;
		for(int i=x;i<=79;i++){
			cout << a[i];
		}
		if(x==80)printf("0\n");
		else printf("\n");
	}
    return 0;
}

C++: #define scanf_s scanf
#define sscanf_s sscanf
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 81
#define _MAX 100
//#define CONSTNUM 10000000
int main(void)
{
	int n, s = 0, x, y, r = 0, a, b;
	int sum_n[50][3] = { 0 };
	char sum[50][_MAX], astr[_MAX], bstr[_MAX], num[11] = { "0123456789" };
	char ca, cb;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%s %s", &astr,_MAX, &bstr, _MAX);
		for (x = 0; astr[x] != '\0'; ++x);
		for (y = 0; bstr[y] != '\0'; ++y);
		if (x > 80 || y > 80) {
			printf("overflow\n");
			continue;
		}
		for (int j = 0; j < x || j < y; ++j) {
			if (j < x && j < y) {
				ca = astr[x - j - 1]; cb = bstr[y - j - 1];
				sscanf_s(&ca, "%d", &a);
				sscanf_s(&cb, "%d", &b);
				if (x > y) sum[i][x - j] = num[(a + b + r) % 10];
				else sum[i][y - j] = num[(a + b + r) % 10];
				if (a + b + r >= 10) r = (a + b + r) / 10;
				else r = 0;
			}
			else {
				if (x > y) {
					if (r != 0) {
						ca = astr[x - j - 1];
						sscanf_s(&ca, "%d", &a);
						sum[i][x - j] = num[(a + r) % 10];
						if (a + r >= 10) r = (a + r) / 10;
						else r = 0;
					}
					else {
						sum[i][x - j] = astr[x - j - 1];
					}
				}
				else {
					if (r != 0) {
						cb = bstr[y - j - 1];
						sscanf_s(&cb, "%d", &b);
						sum[i][y - j] = num[(b + r) % 10];
						if (b + r >= 10) r = (b + r) / 10;
						else r = 0;
					}
					else {
						sum[i][y - j] = astr[y - j - 1];
					}
				}
			}
		}
		sum_n[i][0] = x; sum_n[i][1] = y; sum_n[i][2] = r;
		r = 0;
	}
	for (int i = 0; i < n; ++i) {
		x = sum_n[i][0]; y = sum_n[i][1]; r = sum_n[i][2];
		if (y <= 80 - r && x <= 80 - r) {
			if (r == 1) printf("%d", r);
			for (int j = 1; j <= x || j <= y; ++j) {
				printf("%c", sum[i][j]);
			}
		}
		else {
			printf("overflow");
		}
		printf("\n");
	}
}

C++: #include <iostream>
#include <queue>
#include <vector>
#include <math.h>
#include <algorithm>
#include <iomanip>
#include <string>
#include <stack>
using namespace std;

int main(){
	int n;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		string a, b;
		cin >> a;
		cin >> b;
		if (a.length() > 80 || b.length() > 80) {
			cout << "overflow" << endl;
			continue;
		}

		string result(80, '0');
		int over = 0;
		int j = a.length() - 1;
		int k = b.length() - 1;
		int l = 79;
		for (; l >= 0; --j, --k, --l) {
			int numa = j < 0 ? '0' : a[j];
			int numb = k < 0 ? '0' : b[k];
			int sum = numa + numb - '0' * 2 + over;
			if (sum >= 10) {
				over = 1;
				sum -= 10;
			} else {
				over = 0;
			}
			result[l] = sum + '0';
		}
		if (over == 1) {
			cout << "overflow" << endl;
		} else {
			int j = 0;
			for (; j < result.length() && result[j] == '0'; ++j);
			if (j == result.length()) cout << '0';
			for (; j < result.length(); ++j) cout << result[j];
			cout << endl;
		}
	}

	return 0;
}

C++: #include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int main(void)
{
	int n,s1[10001],s2[10001],s3[10001];
	int i,j,len,len2,x,flg;
	char buf[10001];
	scanf("%d",&n);
	for(j=0;j<n;j++)	{
		for(i=0;i<81;i++)	{
			s1[i]=0;	s2[i]=0;	s3[i]=0;
		}
		scanf("%s",buf);
		len=strlen(buf);
		for(i=len-1;i>=0;i--)	s1[len-1-i]=buf[i]-'0';
		scanf("%s",buf);
		len2=strlen(buf);
		for(i=len2-1;i>=0;i--)	s2[len2-1-i]=buf[i]-'0';		
		for(i=0;i<80;i++)	{
			x=s1[i]+s2[i]+s3[i];
			s3[i]=x%10;
			s3[i+1]=x/10;
		}
		if(s3[80]!=0 || len>80 || len2>80)	{
			printf("overflow\n");
		}
		else {
			flg=0;
			for(i=79;i>=0;i--)	{
				if(flg==1 || s3[i]!=0)	{
					printf("%d",s3[i]);
					flg=1;
				}
			}
			if(flg==0)	{
				printf("0");
			}
			printf("\n");
		}
	}
	return 0;
}

C++: #include <stdio.h>

int main(void) {

  int n;

  char buf[100];
  char c;

  int a[100],b[100];
  int result[100];
  
  int sum,carry;
  int idx1,idx2;
  int maxlen;

  int i,j;

  scanf("%d",&n);

  c=getchar();

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

    fgets(buf,sizeof(buf),stdin);

    idx1=0;

    while(buf[idx1]!='\n') {

      a[idx1]=buf[idx1]-48;

      idx1++;

    }

    //for(j=0;j<idx1;j++) printf("%d",a[j]);

    fgets(buf,sizeof(buf),stdin);

    idx2=0;

    while(buf[idx2]!='\n') {

      b[idx2]=buf[idx2]-48;

      idx2++;

    }

    if(idx1>idx2) maxlen=idx1;
    
    else maxlen=idx2;

    carry=0;

    for(j=0;j<maxlen;j++) {

      if(j<idx1 && j<idx2) sum=a[idx1-j-1]+b[idx2-j-1]+carry;

      else if(j>=idx1) sum=b[idx2-j-1]+carry;

      else sum=a[idx1-j-1]+carry;


      if(sum>=10) carry=1;

      else carry=0;

      result[j]=sum%10;

    }
    //printf("%d\n",carry);
    if(carry==1) {

      if(maxlen>=80) printf("overflow\n");

      else {

	printf("1");

	for(j=maxlen-1;j>=0;j--) {

	  printf("%d",result[j]);

	}

	putchar('\0');

	puts("");

      }

    }

    else {

      for(j=maxlen-1;j>=0;j--) {

	printf("%d",result[j]);

      }

      puts("");

    }

  }

  return 0;

}