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 <stdio.h>

#define MAX 100000

int partition(int [MAX],int,int);

int main(){
  int i,n,q,A[MAX];

  scanf("%d",&n);

  for(i=0; i<n; i++)
    scanf("%d",&A[i]);

  q = partition(A,0,n-1);

  for(i=0; i<n; i++)
    if(i < n-1 && i != q) printf("%d ",A[i]);
    else if(i < n-1 && i == q) printf("[%d] ",A[i]);
    else if(i == n-1) printf("%d\n",A[i]);

  return 0;
}

int partition(int A[MAX],int p,int r){
  int i,j,x,tmp,tmp2;
  x = A[r];
  i = p - 1;
  for(j=p; j<r; j++)
    if(A[j] <= x){
      i++;
      tmp = A[i];
      A[i] = A[j];
      A[j] = tmp;
    }
  tmp2 = A[i+1];
  A[i+1] = A[r];
  A[r] = tmp2;

  return i+1;
}