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++14 (judge verdict: Accepted)

#include <iostream>
#include <string>
#include <cstdio>
#define rep(i,n) Rep(i,0,n)
#define Rep(i,k,n) for(int i=k ; i<n ; i++)
#define rep1(i,n) for(int i=1 ; i <= n ; i++)

using namespace std;

int partition(int a[] , int p, int r){
    int x = a[r];
    int index = p-1;

    Rep(i,p,r){
        if(a[i] <= x){
            index++;
            swap(a[index],a[i]);
        }
    }
    swap(a[index+1],a[r]);

    return index + 1;
}

int main(){
    int n; cin >> n;
    int a[n+10];
    rep1(i,n) cin >> a[i];

    int p = partition(a,1,n);

    rep1(i,n){
        if(i == p) printf("[%d]" , a[i]);
        else printf("%d",a[i]);

        cout << (i != n ? ' ' : '\n'); 
    }
}