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 Go (judge verdict: Accepted)

package main

import (
	"bufio"
	"strconv"
	"fmt"
	"strings"
	"os"
)

func partation(A []int, p, r int) int {
	x := A[r]
	tmp := A[:r]
	i := p - 1
	j := p
	for j < len(tmp) {
		if tmp[j] <= x {
			i++
			tmp[j], tmp[i] = tmp[i], tmp[j] // swap
		}
		j++
	}
	A[r], A[i+1] = A[i+1], x
	return i+1
}

func nextInt(scanner *bufio.Scanner) int {
	scanner.Scan()
	n, err := strconv.Atoi(scanner.Text())
	if err != nil {
		panic(err)
	}
	return n
}

func printArray(Array []int, partation int) {
	tmp := make([]string, len(Array))
	for index, value := range Array {
		if index == partation {
			tmp[index] = "[" + strconv.Itoa(value) + "]"
		} else {
			tmp[index] =  strconv.Itoa(value)
		}
	}
	fmt.Println(strings.Join(tmp, " "))
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	n := nextInt(scanner)
	A := make([]int, n)
	for i := range A {
		A[i] = nextInt(scanner)
	}
	partationIndex := partation(A, 0, n-1)
	printArray(A, partationIndex)
}