Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>

void swap(long *a, long *b);
long partition(long arr[], int low, int high);
void quickSort(long arr[], int low, int high);
void checkRepeat(long arr[], int high);

int main(void){
    int N;
    int bin;
    long *arr;

    bin = scanf("%d", &N);
    arr = malloc((N)*sizeof(long));
    for(int i = 0;i < N;i++)
        bin = scanf("%ld", &arr[i]);
    quickSort(arr, 0, N-1);
    checkRepeat(arr, N-1);
    free(arr);

    return 0;
}

void swap(long *a, long *b){
    long temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

void quickSort(long arr[], int low, int high){
    if(low<high){
        long pi = partition(arr, low, high);

        quickSort(arr, low, pi-1);
        quickSort(arr, pi+1, high);
    }
}

long partition(long arr[], int low, int high){
    long pivot = arr[high];

    int i = low - 1;

    for(int j = low;j <= high;j++){
        if(arr[j] < pivot){
            i++;
            swap(&arr[i], &arr[j]);
        }
    }
    swap(&arr[i+1], &arr[high]);
    return(i+1);
}

void checkRepeat(long arr[], int high){
    int cnt = 1;
    long temp = -1;
    int i;
    for(i = high;i >= 0;i--){
        if(i !=0 && arr[i] == arr[i-1])
            cnt++;
        else if(cnt >= 4 && temp == -1){
            printf("%ld", arr[i]*arr[i]);
            return;
        }
        else if(cnt >= 2){
            if(temp == -1){
                temp = arr[i];
                continue;
            }
            else{
                printf("%ld", temp*arr[i]);
                return;
            }
        }
        else
            cnt = 1;
    }
    printf("0");
}