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: Accepted)

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
import java.util.stream.Collectors;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		List<Long> bars = new ArrayList<>();
		long h = 0;
		long w = 0;

		for (int i = 0; i < n; i++) {
			long x = Integer.parseInt(sc.next());
			bars.add(x);
		}

		Map<Object, Long> result = bars.stream().collect(
				Collectors.groupingBy(x -> x, Collectors.counting()));

		Object[] uniqueBars = bars.stream()
		.distinct().sorted(Comparator.reverseOrder()).toArray();

		for (Object x : uniqueBars) {
			if (result.get(x) >= 4 && (h == 0 && w == 0)) {
				h = (long) x;
				w = (long) x;
				break;
			}
			else if (result.get(x) >= 2 && h == 0) {
				h = (long) x;
				continue;
			} else if (result.get(x) >= 2 && w == 0) {
				w = (long) x;
				continue;
			}
		}

		System.out.println(h * w);
	}
}