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

namespace CsharpSpiral {
    partial class Program {
        static void Main(string[] args) {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = true };
            Console.SetOut(sw);
            new Program().Solve(new ConsoleInput(Console.In, ' '));
            Console.Out.Flush();
        }
        public void Solve(ConsoleInput cin) {
            var n = cin.ReadInt;
            var A = cin.ReadIntArray(n);
            var piv = Partition(A, 0, n - 1);
            var Res = A.Select((x, i) => i == piv ? string.Format("[{0}]", x) : x.ToString()).ToArray().Join();
            Console.WriteLine(Res);
        }
        int Partition(int[] A, int p, int r) {
            var x = A[r];
            var i = p - 1;
            for(int j = p; j < r; j++) {
                if(A[j] <= x) {
                    A.Swap(++i, j);
                }
            }
            A.Swap(i + 1, r);
            return i + 1;
        }
    }

    static class ExtendClass {
        public static void Swap<T>(this T[] X, int i, int j) {
            T tmp = X[i];
            X[i] = X[j];
            X[j] = tmp;
        }
        public static string Join<T>(this T[] X, string separator = " ") => string.Join(separator, X);
        public static S[] Clone<S>(this S[] src) where S : struct {
            var res = new S[src.Length];
            Array.Copy(src, res, src.Length);
            return res;
        }
    }

    class ConsoleInput {
        private readonly TextReader _stream;
        private readonly char _separator = ' ';
        private readonly Queue<string> inputStream;
        public ConsoleInput(TextReader stream, char separator = ' ') {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read {
            get {
                if(inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for(int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) {
            var ret = new string[N];
            for(long i = 0; i < N; ++i)
                ret[i] = Read;
            return ret;
        }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for(long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for(long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }
}