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)

using System;
using System.Text;

namespace ALDS1_6_B
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string[] num = Console.ReadLine().Split(' ');
            int[] list = new int[N];
            for (int i = 0; i < N; i++)
            {
                list[i] = int.Parse(num[i]);
            }

            int pos = partition(list, 0, N - 1);

            string output = ArrayJoin(list, pos);
            Console.WriteLine(output);

            while (true)
            {
                if (string.IsNullOrEmpty(Console.ReadLine())) break;
            }
        }

        public static int partition(int[] arr, int p, int r)
        {
            int x = arr[r];
            int i = p - 1;
            int tmp = 0;
            for (int j = p; j < r; j++)
            {
                if (arr[j] <= x)
                {
                    i++;
                    tmp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = tmp;
                }
            }
            tmp = arr[i + 1];
            arr[i + 1] = arr[r];
            arr[r] = tmp;
            return i + 1;
        }

        public static void swap(int x, int y)
        {
            int tmp = x;
            x = y;
            y = tmp;
        }

        public static string ArrayJoin(int[] arr, int x)
        {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < arr.Length; i++)
            {
                if (sb.Length > 0)
                {
                    sb.Append(" ");
                }
                if (i == x)
                {
                    sb.Append("[");
                    sb.Append(arr[i]);
                    sb.Append("]");
                } else
                {
                    sb.Append(arr[i]);
                }
            }
            return sb.ToString();
        }
    }
}