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)

#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)   for(int i=0;i<(n);++i)
#define SORT(c)    sort((c).begin(),(c).end())
const double EPS = 1e-10;//sample:if((double)>=(double)+EPS)
#define CLR(a) memset((&a), 0 ,sizeof(a))//clear memory
#define dump(x)  cerr << #x << " = " << (x) << endl;//debug
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
//cin,cout release
//std::cin.tie(0);
//ios::sync_with_stdio(false);

using namespace std;

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

int main(){
  int n,A[100000];
  cin>>n;
  for(int i=0;i<n;i++)cin>>A[i];
  int a=partition(A,0,n-1);
  for(int i=0;i<n;i++){
    if(i!=0)cout<<" ";
    if(i==a)cout<<"[";
    printf("%d",A[i]);
    if(i==a)cout<<"]";
  }
  printf("\n");
  return 0;
}