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>

int partition(int *A, int p, int r){
	int i, j = p-1, tmp;

	for(i = p; i < r; i++){
		if(A[i] <= A[r]){
			j++;
			tmp = A[j];
			A[j] = A[i];
			A[i] = tmp;
		}
	}
	tmp = A[j+1];
	A[j+1] = A[r];
	A[r] = tmp;

	return j+1;
}

int main(){
	int n, q, i;
	int *A;

	scanf("%d", &n);
	A = (int *)malloc(sizeof(int) * 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 == q)
			printf("[%d]", A[q]);
		else
			printf("%d", A[i]);
		if(i < n-1)
			printf(" ");
	}
	printf("\n");

	free(A);
	return 0;
}