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.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.Collections;

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

        int n = Integer.parseInt(br.readLine());
        String[] line = br.readLine().split(" ", 0);
        int[] nums = new int[n];
        for (int i = 0; i < n; i++) {
            nums[i] = Integer.parseInt(line[i]);
        }

        partition(nums, 0,n - 1);
    }

    static void printArray(int[] a, int index) {
        for (int i = 0; i < a.length; i++) {
            if (i == index) {
                System.out.print('[');
                System.out.print(a[i]);
                System.out.print(']');
            } else {
                System.out.print(a[i]);
            }

            if (i != a.length - 1) {
                System.out.print(' ');
            }
        }
        System.out.println();
    }

    static int[] swap(int[] a, int i1, int i2) {
        int tmp = a[i1];
        a[i1] = a[i2];
        a[i2] = tmp;
        return a;
    }

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