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 <cmath>
#include <cstdlib>
#include <cstring>

using namespace std;

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 11;
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() {
    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("overflow\n");
            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);
            }
        }

        {
            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];
                    for (int j = 0; temp * pow(10, j) < 10000000; ++j) {
                        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 <bits/stdc++.h>

using namespace std;

int main(){

    string s;
    int n, num1[101], num2[101], num3[102];

    cin >> n;

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

        memset(num1, 0, sizeof(num1));
        memset(num2, 0, sizeof(num2));
        memset(num3, 0, sizeof(num3));

        cin >> s;

        for(int i = 0, len = s.size(); i < len; ++i){
            num1[i] = s[len - 1 - i] - '0';
        }

        cin >> s;

        for(int i = 0, len = s.size(); i < len; ++i){
            num2[i] = s[len - 1 - i] - '0';
        }

        for(int i = 0, num; i < 101; ++i){
            num = num1[i] + num2[i] + num3[i];
            num3[i] = num % 10;
            num3[i + 1] = num / 10;
        }

        int ldigit = 0;
        for(int i = 101; i >= 0; i--){
            if(num3[i] != 0){
                ldigit = i;
                break;
            }
        }

        if((ldigit + 1) >= 80){
            cout << "overflow" << endl;
        } else {
            for(int i = ldigit; i >= 0; i--){
                cout << num3[i];
            }
            cout << endl;
        }

    }

}

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

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	int num1[BUF_SIZE];
	int num2[BUF_SIZE];
	int sum[BUF_SIZE];
	char str[BUF_SIZE];
	int n;
	int i, j, k;
	
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<100; j++) {
			num1[j] = num2[j] = sum[j] = 0;
		}
		
		// first integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
		}
		
		for (j=0; j<strlen(str)-1; j++)  ;
		for (k=0; j>=0; k++) {
			num1[k] = str[j] - '0';
			j--;
		}
		
		// second integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
		}
		
		for (j=0; j<strlen(str)-1; j++)  ;
		for (k=0; j>=0; k++) {
			num2[k] = str[j] - '0';
			j--;
		}
		
		// sum
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j]/10 >= 1) {
				sum[j+1] += sum[j]/10;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] > 0) {
			printf("overflow\n");
		}
	
		for (j=79; sum[j]==0 && j!=0; j--)  ;
		for (; j>=0; j--) {
			printf("%d", sum[j]);
		}
		printf("\n");
	
	}
	
	return 0;
}