Problem:
Partition

Quick sort is based on the Divide-and-conquer approach. In QuickSort(A, p, r), first, a procedure Partition(A, p, r) divides an array A[p..r] into two subarrays A[p..q-1] and A[q+1..r] such that each element of A[p..q-1] is less than or equal to A[q], which is, inturn, less than or equal to each element of A[q+1..r]. It also computes the index q.

In the conquer processes, the two subarrays A[p..q-1] and A[q+1..r] are sorted by recursive calls of QuickSort(A, p, q-1) and QuickSort(A, q+1, r).

Your task is to read a sequence A and perform the Partition based on the following pseudocode:

Partition(A, p, r)
1 x = A[r]
2 i = p-1
3 for j = p to r-1
4 do if A[j] <= x
5 then i = i+1
6 exchange A[i] and A[j]
7 exchange A[i+1] and A[r]
8 return i+1

Note that, in this algorithm, Partition always selects an element A[r] as a pivot element around which to partition the array A[p..r].

Input

The first line of the input includes an integer n, the number of elements in the sequence A.

In the second line, Ai (i = 1,2,...,n), elements of the sequence are given separated by space characters.

Output

Print the sorted sequence. Two contiguous elements of the sequence should be separated by a space character. The element which is selected as the pivot of the partition should be indicated by [  ].

Constraints

1 ≤ n ≤ 100,000

0 ≤ Ai ≤ 100,000

Sample Input 1

12
13 19 9 5 12 8 7 4 21 2 6 11

Sample Output 1

9 5 8 7 4 2 6 [11] 21 13 19 12

Solution in JAVA (judge verdict: Accepted)

import java.util.*;

class Main {
    public static void main(String[] args) {
        new Main().main();
    }

    public void main() {
        Scanner scanner = new Scanner(System.in);
        int n = Integer.parseInt(scanner.nextLine());
        String[] line = scanner.nextLine().split(" ", n);

        int[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = Integer.parseInt(line[i]);
        }

        int index = this.partition(arr, 0, n-1);
        for (int i = 0; i < n; i++) {
            if (i == index) {
                System.out.printf("[%d]", arr[i]);
            } else {
                System.out.print(arr[i]);
            }

            if (i == n - 1) {
                System.out.print("\n");
            } else {
                System.out.print(" ");
            }
        }
    }

    private int partition(int[] arr, int p, int r) {
        int x = arr[r];
        int i = p - 1;

        for (int j = p; j < r; j++) {
            if (arr[j] <= x) {
                i++;
                int tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
            }
        }

        int tmp = arr[i+1];
        arr[i+1] = arr[r];
        arr[r] = tmp;
        return i + 1;
    }
}