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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)


import java.io.*;
import java.util.*;
//import java.util.Map.Entry;
//import java.util.stream.*;
//import java.math.*;

public class Main {

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
        Scanner sc = new Scanner(System.in);
        long MOD = (long) (Math.pow(10, 9) + 7);
        int n = sc.nextInt();
        long array[] = new long[n];
        long max = 0, min = 0;
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextLong();
        }
        Arrays.sort(array);
        int count = 0;
        int i = array.length - 1;
        while (i >= 0 && true) {
            long num = array[i];
            count = 1;
            while (true) {
                if (i > 0 && num == array[i - 1]) {
                    i--;
                    count++;
                } else {
                    i--;
                    break;
                }
            }
            if (count >= 4) {
                max = num;
                min = num;
                break;
            } else if (count >= 2 && max == 0) {
                max = num;
            } else if (count >= 2 && max > 0) {
                min = num;
                break;
            }
        }
        if (max > 0 && min > 0) {
            System.out.println(max * min);
        } else {
            System.out.println("0");
        }
        bw.flush();
    }
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    static class Pair implements Comparable<Pair> {

        int a, b;

        public Pair(int x, int y) {
            a = x;
            b = y;
        }

        @Override
        public int compareTo(Pair p) {
            return a - p.a;
        }
    }

    static void forl_0(int n, int[] array, Scanner sc) {
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
    }

    static void forl_1(int n, int[] array, Scanner sc) {
        for (int i = 1; i <= n; i++) {
            array[i] = sc.nextInt();
        }
    }

    static void two_d(int n, int m, int[][] array, Scanner sc) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                array[i][j] = sc.nextInt();
            }
        }
    }

    static boolean isPrime(long n) {
        if (n < 2) {
            return false;
        } else if (n == 2 || n == 3) {
            return true;
        } else if (n % 2 == 0 || n % 3 == 0) {
            return false;
        }
        long sqrtN = (long) Math.sqrt(n) + 1;
        for (long i = 6L; i <= sqrtN; i += 6) {
            if (n % (i - 1) == 0 || n % (i + 1) == 0) {
                return false;
            }
        }
        return true;
    }

    public static long gcd(long a, long b) {
        return (b == 0) ? a : gcd(b, a % b);
    }

    public static long lcm(long a, long b) {
        return a / gcd(a, b) * b;
    }
}