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 <cstdio>
#include <cstdlib>
using namespace std;

int main(){
	int n;
	cin >> n;
	for(int j=0;j<n;j++){
		char a[81],b[81],c[81];
		int ka = 0;
		int kb = 0;
		int kc = 0;
		bool ovf = false;
		cin >> a[ka];
		ka++;
		while(1){
			scanf("%c",&a[ka]);
			if(ka==80 and a[ka]!='\n') ovf = true;
			if(a[ka]=='\n' or ovf) break;
			ka++;
		}
		while(1){
			scanf("%c",&b[kb]);
			if(kb==80 and b[kb]!='\n') ovf = true;
			if(b[kb]=='\n' or ovf) break;
			kb++;
		}
		int k = 0;
		char t;
		for(int i=0;i<81;i++){
			if(i==80 and k==1) ovf = true;
			if((ka<=i and kb<=i) or ovf) break;
			int ia = 0;
			t = a[ka-i-1];
			if(ka>=i) ia = atoi(&t);
			int ib = 0;
			t = b[kb-i-1];
			if(kb>=i) ib = atoi(&t);
			int s = ia + ib + k;
			k = 0;
			if(s>9){ k = 1; s = s%10;}
			itoa(s,&c[i],10);
			kc++;
		}
		if(ovf) cout << "overflow" << endl;
		else{
			for(int i=kc-1;i>=0;i--) cout << c[i];
			cout << endl;
		}
	}
}

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

string add(string a,string b){
    string res;
    int sa=a.size();
    int sb=b.size();
    int va,vb,t=0;
    for(int i=1;i<=max(sa,sb);i++){
        if(sa-i<0)va=0;else va=a[sa-i]-'0';
        if(sb-i<0)vb=0;else vb=b[sb-i]-'0';
        if(t)va++;
        t=((va+vb)>9);res=(char)(va+vb-t*10+'0')+res;
    }
    if(t)res="1"+res;
    return res;
}

int main() {
    int n;
    cin >> n;
    while(n){
        string a, b;
        string ans;
        cin >> a >> b;
        ans = add(a,b);
        if(ans.size()>80){
            cout << "overflow" << endl;
        }else{
            cout << ans << endl;
        }
    }
    return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b;
    cin >>a>>b;
    int num[900] = {0},tab = 0,i = 0;
    int as = a.size()-1;
    int bs = b.size()-1;
    int nmi = min(as,bs);
    int nma = max(as,bs);
    cout <<nma<<endl;
    for(int x; i<=nma; i++){
      if(i<=nmi){x = tab+a[as-i]+b[bs-i]-'0'*2;}
      else if(as<bs){x = tab+b[bs-i]-'0';}
      else if(bs<as){x = tab+a[as-i]-'0';}
      tab = x/10;
      num[i] = x%10;
    }
    cout<<i<<endl;
    if(tab){num[nma+1] = tab;i++;}
    if(i>80){cout <<"overflow";}
    else{for(int x=i-1; x>=0; x--){cout <<num[x];}}
    cout <<endl;
  }
  return 0;
}

C++: #include <cstdio>
#include <map>
using namespace std;

int get_length(char *data) {
	for (int i = 0;; i++) {
		if (data[i] == '\0') {
			return i;
		}
	}
	return 0;
}

bool is_more_ten(char *d) {
	if (d[0] == 'X' || d[0] == 'A' || d[0] == 'B' || d[0] == 'C' ||
		d[0] == 'D' || d[0] == 'E' || d[0] == 'F' || d[0] == 'G' ||
		d[0] == 'H' || d[0] == 'I') {
		return true;
	}
	return false;
}

void shift(char *data, int length) {
	for (int i = length; i != -1; i--) {
		data[i+1] = data[i];
	}
}

int main(int argc, char *argv[]) {
	int dataN;
	
	map<char, int> chr2int;
	chr2int.insert(map<char, int>::value_type('0',0));
	chr2int.insert(map<char, int>::value_type('1',1));
	chr2int.insert(map<char, int>::value_type('2',2));
	chr2int.insert(map<char, int>::value_type('3',3));
	chr2int.insert(map<char, int>::value_type('4',4));
	chr2int.insert(map<char, int>::value_type('5',5));
	chr2int.insert(map<char, int>::value_type('6',6));
	chr2int.insert(map<char, int>::value_type('7',7));
	chr2int.insert(map<char, int>::value_type('8',8));
	chr2int.insert(map<char, int>::value_type('9',9));
	chr2int.insert(map<char, int>::value_type('X',10));
	chr2int.insert(map<char, int>::value_type('A',11));
	chr2int.insert(map<char, int>::value_type('B',12));
	chr2int.insert(map<char, int>::value_type('C',13));
	chr2int.insert(map<char, int>::value_type('D',14));
	chr2int.insert(map<char, int>::value_type('E',15));
	chr2int.insert(map<char, int>::value_type('F',16));
	chr2int.insert(map<char, int>::value_type('G',17));
	chr2int.insert(map<char, int>::value_type('H',18));
	chr2int.insert(map<char, int>::value_type('I',19));
	
	map<char, int> end_chr2int;
	end_chr2int.insert(map<char, int>::value_type('X',0));
	end_chr2int.insert(map<char, int>::value_type('A',1));
	end_chr2int.insert(map<char, int>::value_type('B',2));
	end_chr2int.insert(map<char, int>::value_type('C',3));
	end_chr2int.insert(map<char, int>::value_type('D',4));
	end_chr2int.insert(map<char, int>::value_type('E',5));
	end_chr2int.insert(map<char, int>::value_type('F',6));
	end_chr2int.insert(map<char, int>::value_type('G',7));
	end_chr2int.insert(map<char, int>::value_type('H',8));
	end_chr2int.insert(map<char, int>::value_type('I',9));
	
	const char int2chr[] = {
		'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 
		'X', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I'
	};
	
	fscanf(stdin, "%d", &dataN);
	for (int i = 0; i < dataN; i++) {
		char buf1[1000], buf2[1000], sum[1000];
		int buf1_cnt, buf2_cnt, longer;
		// scan
		fscanf(stdin, "%s", buf1);
		fscanf(stdin, "%s", buf2);
		buf1_cnt = get_length(buf1);
		buf2_cnt = get_length(buf2);
		// is overflow ?
		if (buf1_cnt > 80 || buf2_cnt > 80) {
			puts("overflow");
			continue;
		}
		
		if (buf1_cnt > buf2_cnt) {
			sum[buf1_cnt] = '\0';
			longer = buf1_cnt;
			int idx = buf1_cnt-1;
			int diff = buf1_cnt - buf2_cnt;
			for (int i = buf2_cnt-1; i != -1 ; i--) {
				int t_sum = chr2int[buf1[idx]] + chr2int[buf2[i]];
				if (t_sum > 9) {
					buf1[idx-1] = int2chr[chr2int[buf1[idx-1]]+1];
					t_sum %= 10;
				}
				sum[idx] = int2chr[t_sum];
				idx--;
			}
			for (int i = diff-1; i != -1; i--) {
				int t_sum = chr2int[buf1[i]];
				if (i != 0) {
					if (t_sum > 9) {
						buf1[i-1] = int2chr[chr2int[buf1[i-1]]+1];
						t_sum %= 10;
					}
				}
				sum[i] = int2chr[t_sum];
			}
		}
		else if (buf1_cnt < buf2_cnt) {
			longer = buf2_cnt;
			sum[buf2_cnt] = '\0';
			int idx = buf2_cnt-1;
			int diff = buf2_cnt - buf1_cnt;
			for (int i = buf1_cnt-1; i != -1 ; i--) {
				int t_sum = chr2int[buf2[idx]] + chr2int[buf1[i]];
				if (t_sum > 9) {
					buf2[idx-1] = int2chr[chr2int[buf2[idx-1]]+1];
					t_sum %= 10;
				}
				sum[idx] = int2chr[t_sum];
				idx--;
			}
			for (int i = diff-1; i != -1; i--) {
				int t_sum = chr2int[buf2[i]];
				if (i != 0) {
					if (t_sum > 9) {
						buf2[i-1] = int2chr[chr2int[buf2[i-1]]+1];
						t_sum %= 10;
					}
				}
				sum[i] = int2chr[t_sum];
			}
		}
		else {
			longer = buf1_cnt;
			sum[buf2_cnt] = '\0';
			int idx = buf2_cnt-1;
			int diff = buf2_cnt - buf1_cnt;
			for (int i = buf1_cnt-1; i != -1 ; i--) {
				int t_sum = chr2int[buf1[idx]] + chr2int[buf2[i]];
				if (t_sum > 9) {
					if (i != 0) {
						buf2[idx-1] = int2chr[chr2int[buf2[idx-1]]+1];
						t_sum %= 10;
					}
				}
				sum[idx] = int2chr[t_sum];
				idx--;
			}
		}
		// check most left number
		if (is_more_ten(sum)) {
			if (buf1_cnt == 80 || buf2_cnt == 80) {
				puts("overflow");
				continue;
			}
			shift(sum, longer);
			sum[0] = '1';
			sum[1] = int2chr[end_chr2int[sum[1]]];
		}
		printf("%s\n", sum);
	}
	
	return 0;
}