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.Linq;
using System.Collections.Generic;
using static System.Console;
using System.Text;
using System.IO;

namespace AOJ
{
    class Program
    {
        static public long[] Sarray() { return ReadLine().Split().Select(long.Parse).ToArray(); }
        static public List<long> Slist() { return ReadLine().Split().Select(long.Parse).ToList(); }
        static long cnt = 0;
        static void Main(string[] args)
        {
            var N = Sarray()[0];
            var Ai = Sarray();
            var pi = partition(Ai, 0, (int)N - 1);
            StringBuilder sb=new StringBuilder();
            for (var i = 0; i < N; ++i)
            {
                if (i != pi)
                { sb.Append($"{Ai[i]} "); }
                else
                { sb.Append($"[{Ai[i]}] "); }
            }
            WriteLine(sb.ToString().Trim());
        }
        static void swap<T>(ref T  a,ref T b)
        {
            var t = a;
            a = b;
            b = t;
        }
        static int partition(long[] A,int p, int r)
        {
            var x = A[r];
            var i = p - 1;
            for (var j = p; j < r;++j)
            {
                if(A[j]<=x)
                {
                    ++i;
                    swap<long>(ref A[i], ref A[j]);
                }
            }
            swap<long>(ref A[i + 1], ref A[r]);
            return i + 1;
        }
    }
}