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 N 100000
 
int arr[N];
 void sw(int *x, int *y){
  int tmp;
  tmp = *x;
  *x = *y;
  *y = tmp;
}
void part(int P, int R){
  int x = arr[R];
  int i = P - 1, j;
  for(j = P;j < R;j++){
    if(arr[j] <= x){
      i++;
      sw(&arr[i], &arr[j]);
    }
  }
  sw(&arr[i+1], &arr[R]);
  for(j = P;j <= R;j++){
    if(j != P) printf(" ");
    if(j == i+1) printf("[%d]", arr[j]);
    else printf("%d", arr[j]);
  }
  printf("\n");
}
 
int  main(int argc,char *argv[]){
  int n, i;
  scanf("%d", &n);
  for(i = 0;i < n;i++) scanf("%d", &arr[i]);
  part(0, n-1);

  return 0;
}