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>

void swap(int *A, int i, int j){
    
    int temp = A[i];
    A[i] = A[j];
    A[j] = temp;
}

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

/*
 * 
 */
int main(int argc, char** argv) {

    int p = 0, n, track; 
    scanf("%d",&n);
    int *A = (int*)malloc(n * sizeof(int));
    for(int a = 0; a < n; a++){
            scanf("%d",&A[a]);
    }
    track = Partition(A,0,n-1);
    
    for(int a = 0; a < n; a++){
        
            if(a == track)printf("[");
            printf("%d",A[a]);
            if(a == track)printf("]");
            if(a < n - 1)printf(" ");
            
    }
    printf("\n");
    return (0);
}