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.io.BufferedReader;
//import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) throws NumberFormatException, IOException {
		//System.setIn(new FileInputStream("./input.txt"));
		
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());

		String[] t = br.readLine().split(" ");
		int[] a = new int[n];
		for(int i = 0; i < n; ++i){
			a[i] = Integer.parseInt(t[i]);
		}
		int s = Partition(a, 0, n - 1);
		
		StringBuilder sb = new StringBuilder();
		for(int i = 0; i < n; ++i) {
			if(i != 0){
				sb.append(" ");
			}
			if(i == s){
				sb.append('[');
				sb.append(a[i]);
				sb.append(']');
			}else{
				sb.append(a[i]);
			}
		}
		System.out.println(sb);
		
	}
	public static int Partition(int[] a, int p, int r){
		 int x = a[r];
		 int i = p - 1;
		 int tmp;
		 for(int j = p; j < r; ++j) {
			 if(a[j] <= x){
				 i += 1;
				 tmp = a[i];
				 a[i] = a[j];
				 a[j] = tmp;
			 }
		 }
		 tmp = a[i + 1];
		 a[i + 1] = a[r];
		 a[r] = tmp;
		 return i + 1;
	}
}