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 main(){

    int n, i, j, k;
    int x, t;
    int *a;

    scanf( "%d", &n );

    a = (int*)malloc(sizeof(int)*n);

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


    x = a[n-1];
    i=-1;

    for( j=0; j<(n-1); j++ ){
        if( a[j] <= x ){
            i++;
            t = a[i];
            a[i] = a[j];
            a[j] = t;
        }
    }
    i++;
    t = a[i]; a[i]=a[n-1]; a[n-1]=t;

    for( k=0; k<n; k++ ){
        if( k == i ) printf( "[" );
        printf( "%d", a[k]  );
        if( k == i ) printf( "]" );
        printf( "%c", ((n-1>k))?' ':'\n' );
    }
    

    free( a );
    return 0;
}