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: WA: Presentation Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Aoj.ALDS.Chapter6B
{
    class Program
    {
        /// <summary>
        /// partition??§??????????????????????????????
        /// ??????????????????????´?????????????????????\??????
        /// ??????????????????????´???????????????????????????§?????????????????????array??????????????????
        /// (array???????????´?£???????????????????)???
        /// </summary>
        /// <param name="array"></param>
        /// <param name="first">??°??????????????????????´??????????</param>
        /// <param name="last">??°??????????????????????´???????(??????)???</param>
        /// <param name="partition">partition??¨??????????????????????´??????????</param>
        /// <returns>partition??????????±??????????????´??????????</returns>
        public static int Partition<T>(List<T> array, int first, int last, int partition = -1)
        where T : IComparable<T>
        {
            if (first < 0 || last >= array.Count)
                throw new System.ArgumentException("");

            if (partition == -1)
                // ?????????????????§???????°????????´?????????????????????£?????§???????????????
                partition = last;

            // ???????????£?????§???????±???¨?????????(last?????????????§????????????????)???
            T x = array[partition];
            Swap(array, partition, last);

            int i = first;
            for (int j = first; j < last; j++)
            {
                if (array[j].CompareTo(x) <= 0)
                {
                    Swap(array, i, j);
                    // ?????°i???Swap???????????´??°???????????????????????????????????¨????¢?????????????????????????????????????
                    i++;
                }
            }

            // ????°?????????????????????????x?????????????????£?????§??????????±??????????Swap???
            Swap(array, i, last);

            return i;
        }

        private static void Swap<T>(List<T> array, int i, int j)
        {
            T tmp = array[i];
            array[i] = array[j];
            array[j] = tmp;
        }

        public static void Solve()
        {
            int n = int.Parse(Console.ReadLine());
            var list = Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse).ToList();

            int pIndex = Partition(list, 0, n -1);
            for (int i = 0; i < n; i++)
            {
                if (i == pIndex)
                {
                    Console.Write("[{0}] ", list[i]);
                }
                else
                {
                    Console.Write("{0} ", list[i]);
                }
            }
            Console.WriteLine("");
        }
        public static void Main()
        {
            // Console.SetIn(new System.IO.StreamReader("Aoj/AlgorithmsAndDataStructures/Input/06_Sort_B_01.txt"));
            Solve();
        }
    }
}