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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)


import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0 ; i < n; i++) a[i] = sc.nextInt();
		Map<Integer, Integer> map = new HashMap<>();
		for(int i = 0 ; i < n ; i++) {
			if(map.containsKey(a[i])) {
				map.put(a[i], map.get(a[i]) + 1);
			} else {
				map.put(a[i], 1);
			}
		}
		List<Entry<Integer, Integer>> entry = new ArrayList<>(map.entrySet());
		Collections.sort(entry, new Comparator<Entry<Integer, Integer>>() {
			public int compare(Entry<Integer, Integer> obj1, Entry<Integer, Integer> obj2) {
				if(obj1.getValue() >= 2 && obj2.getValue() >= 2) return obj2.getKey() - obj1.getKey();
				else return obj2.getValue() - obj1.getValue();
			}
		});
		if(entry.get(0).getValue() >= 4) {
			System.out.println(Math.max(entry.get(0).getKey() * entry.get(0).getKey(), entry.get(0).getKey() * entry.get(1).getKey()));
		} else if(entry.get(0).getValue() >= 2 && entry.get(1).getValue() >= 2) {
			System.out.println(entry.get(0).getKey() * entry.get(1).getKey());
		} else {
			System.out.println(0);
		}
	}
}