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: Wrong Answer)

using System;

namespace ALDS1_6_B
{
    static class Swapper
    {
        public static void Integer(ref int a, ref int b)
        {
            if (a == b) return;
            a ^= b;
            b ^= a;
            a ^= b;
        }
    }
    static class Algorithm
    {
        public delegate void SwapperAction<T>(ref T a, ref T b);
        public static int Partition<T>(T[] a, int s, int t, SwapperAction<T> swapper) where T : IComparable
        {
            int p = s;
            T x = a[t];
            for (int i = s; i < t; ++i)
            {
                if (a[i].CompareTo(x) < 0) swapper.Invoke(ref a[p++], ref a[i]);
            }
            swapper.Invoke(ref a[p], ref a[t]);
            return p;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Console.ReadLine();
            var a = Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
            int q = Algorithm.Partition(a, 0, a.Length - 1, Swapper.Integer);
            var strs = Array.ConvertAll(a, (s) => s.ToString());
            strs[q] = '[' + strs[q] + ']';
            Console.WriteLine(string.Join(" ", strs));
        }
    }
}