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.*;
import java.io.*;

class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		int[] a = new int[n];

		String s = br.readLine();
		String[] str = s.split(" ");
		for (int i=0; i<n; i++) {
			a[i] = Integer.parseInt(str[i]);
		}

		int p = partition(a,0,n-1);

		StringBuilder sb = new StringBuilder();
		for (int i=0; i<n-1; i++) {
			if(i != p){
				sb.append(a[i]).append(" ");
			}else {
				sb.append("[").append(a[i]).append("] ");
			}
		}
		if (n-1 != p) {
			System.out.println(sb.append(a[n-1]));
		}else {
			System.out.println(sb.append("[").append(a[n-1]).append("]"));
		}

		br.close();
	}

	static int partition(int[] a,int p,int r) {
		int x = a[r];
		int i = p-1;
		for (int j=p; j<r; j++) {
			if (a[j] <= x) {
				i = i+1;
				swap(a,i,j);
			}
		}
		swap(a,i+1,r);
		return i+1;
	}

    static void swap(int[] a,int idx1,int idx2){
        int temp = a[idx1];
        a[idx1] = a[idx2];
        a[idx2] = temp;
    }

}