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>
#include<stdlib.h>
#define N 1e5

int partition(int*, int, int);

int partition(int *A, int p, int r){

  int x=A[r], i=p-1, j, tmp;

  for(j=p;j<r;j++){

    if(A[j]<=x){

      i++;

      tmp=A[i];

      A[i]=A[j];

      A[j]=tmp;

    }

  }tmp=A[i+1];

  A[i+1]=A[r];

  A[r]=tmp;

  return i+1;

}

int main(){

  int i, n, q;

  int *A=(int*)malloc(sizeof(int)*N);

  scanf("%d", &n);

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

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

  printf("%d", A[0]);

  for(i=1;i<n;i++){

    if(i==q) printf(" [%d]",A[i]);

    else printf(" %d", A[i]);

  }printf("\n");

  free(A);

  return 0;

}