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 <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <cstdio>
#include <cmath>
#include <tuple>
#include <iomanip>
#include <numeric>
#include <unordered_map>
#include <sstream>   
#include<limits.h>
#include<list>
using namespace std;
#define i64  long long
#define int  long long
 
#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807
#define MOD 1000000007

int M_pertition(int p,int r, vector<int> & vec, int N)
{
  //p先頭 r末尾
  int x,i,j,t;
  x = vec[r];
  i = p-1;
  for (j = p; j < r; j++)
  {
    /* code */
    if(vec[j] <= x)
    {
      i++;
      t = vec[i];
      vec[i] = vec[j];
      vec[j] = t;
    }
  }

  t = vec[i+1]; vec[i+1] = vec[r]; vec[r] = t;//最後に末尾と入れ替え

  return i + 1;
  
}
void solve()
{
  int N;
  cin>>N;
  vector<int> vec(N,0);
  for (int i = 0; i < N; i++)
  {
    cin>>vec[i];//最後の要素でパーティションを作る
  }
  
  int q = M_pertition(0,N-1,vec,N);
  for (int i = 0; i < N; i++)
  {
    if(i) cout<<" ";
    if(i == q) cout<<"[";
    cout<<vec[i];
    if(i==q) cout<<"]";
  }
  cout<<endl;
  
  return;  
}

int32_t main(int32_t argc, const char *argv[])
{
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(8);

  solve();

 
  return 0;
}