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 C# (judge verdict: Accepted)

// Partition.cs

using System;

class Partition {
	public static void Main() {

		int n = int.Parse(Console.ReadLine());

		string[] str = Console.ReadLine().Split(' ');

		int[] a = new int[n];
		for(int i = 0; i < n; i++) {
			a[i] = int.Parse(str[i]);
		}

		int r = a[a.Length - 1], p = 0, q = 0, swap;

		for(int i = 0; i < n; i++) {

			if(r >= a[i]) {
				swap = a[p];
				a[p] = a[i];
				a[q] = swap;
				p++;
				q++;
			} else if(i == n - 1) {
				swap = a[p];
				a[p] = a[i];
				a[n - 1] = swap;
			} else {
				a[q] = a[i];
				q++;
			} 

		}
		
		for(int i = 0; i < n; i++) {
			if(a[i] == r && p - 1 == i) {
				Console.Write("[" + a[i] + "] ");
			} else if(i != n - 1) {
				Console.Write(a[i] + " ");
			} else {
				Console.WriteLine(a[i]);
			}
		}

	}
}