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.Scanner;
import java.util.Map;
import java.util.TreeMap;
import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        Map<Integer,Integer> a = new TreeMap<>();
        for(int i = 0;i < n;i++){
            int x = sc.nextInt();
            if(a.get(x) != null){
                a.put(x,a.get(x)+1);
                continue;
            }
            a.put(x,1);
        }
        List <Integer> list = new ArrayList<>(a.keySet());
        Collections.sort(list,Collections.reverseOrder());
        //System.out.println(list);
        int edge1 = 0;
        int edge2 = 0;
        for(Integer it : list){
            //System.out.println("it = " + it);
            if(a.get(it) < 2){
                continue;
            }
            else if(a.get(it) >= 2 && a.get(it) < 4){
                if(edge1 == 0){
                    edge1 = it;
                    continue;
                }
                edge2 = it;
                break;
            }
            else if(a.get(it) >= 4){
                if(edge1 != 0){
                    edge2 = it;
                    break;
                }
                edge1 = it;
                edge2 = it;
                break;
            }
        }
        System.out.println(edge1 * edge2);
    }
}