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)

//パーティション
//2018-12-21
//Ebitani Akira

#include <stdio.h>
#include <stdlib.h>

#define N 100000

int Array[N];
int n;

int main (){
  int i, p;

  scanf( "%d", &n );
  if( n < 1 || n > 100000 ){
    printf( "要素数が適当ではありません。\n" );
    exit(1);
  }
  for( i = 0 ; i < n ; i++ ){
    scanf( "%d", &Array[i] );
  }

  p = partition( 0, n - 1 );

  for( i = 0 ; i < n ; i++ ){
    if( i == p ){
      printf( "[" );
    }
    
    printf( "%d", Array[i] );
    
    if( i == p ){
      printf( "]" );
    }


    if( i != n - 1 ){
      printf( " " );
    }
  }
  printf( "\n" );
  return 0;
}

int partition( int a, int b ){
  int A, i, j, temp;

  A = Array[b];
  i = a - 1;

  for( j = a ; j < b ; j++ ){
    if( Array[j] <= A ){
      i++;
      temp = Array[i];
      Array[i] = Array[j];
      Array[j] = temp;
    }
  }

  temp = Array[i+1];
  Array[i+1] = Array[b];
  Array[b] = temp;

  return i + 1;
}