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.Scanner;

/**
 * @author kawakami
 *
 */
class Main {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner as = new Scanner(System.in);
		StringBuffer sb = new StringBuffer();
		
		int x = 0;
		int n = 0;
		int i = 0;
		int j = 0;
		int p = 0;
		int r = 0;
		int tre = 0;
		
		int A[];
		
		n = Integer.parseInt(as.next());
		
		A = new int[n];
		
		for(i=0;i<n;i++){
			
			A[i] = Integer.parseInt(as.next());
			
		}
		
		p = 0;
		r = n-1;
		
		x = A[r];
		i = p-1;
		
		for(j=p;j<r;j++){
			
			if(A[j] <= x){
				
				i = i + 1;
				tre = A[i];
				A[i] = A[j];
				A[j] = tre;
				
			}
			
		}
		
		tre = A[i + 1];
		A[i + 1] = A[r];
		A[r] = tre;
		
		//System.out.println(i+1);
		for(int k=0;k<n;k++){
			
			
			//System.out.println(k+":"+(i+1));
			if(k == (i+1)){
				
				//System.out.println("!!");
				sb.append("[").append(String.valueOf(A[k])).append("]").append(" ");
				
			}
			else if(k != n-1){
				
				sb.append(String.valueOf(A[k])).append(" ");
				
			}
			else{
				
				sb.append(String.valueOf(A[k]));
				
			}
			
		}
		
		System.out.println(sb);

	}

}