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<string>
#include<cstdio>
#include<stack>
#include<algorithm>
using namespace std;
string str1;
string str2;
stack<int> sta;

int main()
{
  int n;
  int i,j;
  int exc;//??°?????????
  int sum;

  cin>>n;
  for(i=0;i<n;i++){
    cin>>str1;
    cin>>str2;
    if(str1.size()>=80 || str2.size()>=80){
      cout<<"overflow"<<endl;
      return 0;
    }
    exc=0;
    for(j=1;j<=min(str1.size(),str2.size());j++){
      sum=str1[str1.size()-j]-'0'+str2[str2.size()-j]-'0'+exc;
      exc=sum/10;
      sta.push(sum%10);
    }
    if( str1.size() < str2.size() ){
      while(j<=str2.size()){
	sum=str2[str2.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }else if( str1.size() > str2.size() ){
      while(j<=str1.size()){
	sum=str1[str1.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }
    if(max(str1.size(),str2.size())==79 && exc==1){
      cout<<"overflow"<<endl;
      return 0;
    }
    if(exc)
      cout<<exc;
    for(j=0;j<max(str1.size(),str2.size());j++){  
      cout<<sta.top();
      sta.pop();
    }
    cout<<endl;
  }
}

C++: #include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 20;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s\n%s", &inputA, &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN <= lenA) || (MAX_INPUT_LEN <= lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if ((9999999 <= r[2]) || (0 < r[1])) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    bool didFor = false;
                    for (int j = 0; temp * pow(10, j) < 1000000; ++j) {
                        printf("0");
                        didFor = true;
                    }
                    if (didFor) {
                        printf("0");
                    }
                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");
        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }
    return 0;
}

C++: #include<stdio.h>
#include<math.h>
#include<string.h>

int main(void){

	int N,math1,math2,kuri,keta;
	int i,j,k;
	char fi[100],se[100],ans[100];

	scanf("%d",&N);

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

		scanf("%s",&fi);
		scanf("%s",&se);

		
		for(j=strlen(fi)-1,k=strlen(se)-1,keta=0,kuri=0;j>=0||k>=0;j--,k--,keta++){
			if(j<0) math1=0;
			else math1=fi[j]-'0';
			if(k<0) math2=0;
			else math2=se[k]-'0';
            /*ans[keta]=((math1+math2+kuri)%10)+'0';*/
			kuri=(math1+math2+kuri)/10;
		}
		if(kuri==1) printf("1");
		if(keta-1<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		printf("\n");
	}

	return 0;
}