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"
	"bytes"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var sc = bufio.NewScanner(os.Stdin)

func scanInt() int {
	num, _ := strconv.Atoi(scanString())
	return num
}

func scanString() string {
	sc.Scan()
	return sc.Text()
}

func init() {
	sc.Split(bufio.ScanWords)
}

func partition(A []int, p, r int) int {
	x := A[r]
	i := p - 1
	for j := p; j < r; j++ {
		if A[j] <= x {
			i++
			A[i], A[j] = A[j], A[i]

		}
	}
	A[i+1], A[r] = A[r], A[i+1]
	return i + 1
}

func main() {
	n := scanInt()
	A := make([]int, n)
	for i := range A {
		A[i] = scanInt()
	}

	index := partition(A, 0, n-1)
	buf := bytes.Buffer{}
	for i, e := range A {
		if i == index {
			buf.WriteString("[")
			buf.WriteString(strconv.Itoa(e))
			buf.WriteString("]")
		} else {
			buf.WriteString(strconv.Itoa(e))
		}
		buf.WriteString(" ")
	}
	fmt.Println(strings.TrimRight(buf.String(), " "))
}