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>
 
int Partition(int *,int,int);
void swap(int *,int,int);
 
int main(){
  int i,n;
  int ce;
  int a[100000];
 
  scanf("%d",&n);
 
  for(i=0;i<n;i++) scanf("%d",&a[i]);
 
  ce = Partition(a,0,n-1);
 
  for(i=0;i<n-1;i++){
    if(i == ce) printf("[%d] ",a[i]);
    else printf("%d ",a[i]);
  }
  if(i == ce)printf("[%d]\n",a[i]);
  else printf("%d\n",a[i]);
 
  return 0;
}

int Partition(int *a,int p,int r){
  int i,j,x,box;
 
  x=a[r];
  j=p-1;
 
  for(i=p;i<r;i++){
    if( a[i] <= x){
      j++;
      swap(a,j,i);
    }
  }
  swap(a,j+1,r);
 
  return j+1;
}
 
void swap(int *a,int p,int q){
  int box;
 
  box = a[p];
  a[p]=a[q];
  a[q]=box;
}