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>
#include <string.h>
#include <ctype.h>
#include <math.h>

//============================================================================
#define pred(x) ((x)-1)
#define succ(x) ((x)+1)
//============================================================================
//----------------------------------------------------------------------------
void exchange(long *x, long *y)
{
    long z;
    z=*x, *x=*y, *y=z;
}
//----------------------------------------------------------------------------
long Partition(long *A, long p,  long r)
{
    long i,j,x;

    x = A[r];
    i = p-1;
    for (j = p; j<r; j++)
        if (A[j] <= x){
            i++;
            exchange(&A[i], &A[j]);
        }
    exchange(&A[i+1], &A[r]);
    return i+1;
}
//----------------------------------------------------------------------------
/*
12
13 19 9 5 12 8 7 4 21 2 6 11
*/
//----------------------------------------------------------------------------
//----------------------------------------------------------------------------
void input(FILE *file)
{
    long i,n,x; long *A;

    fscanf(file, "%ld", &n);
    A = malloc(succ(n)*sizeof(long));
    for (i=0; i<n; i++){
        fscanf(file, "%ld", &A[i]);
    }
    x=Partition(A, 0, pred(n));
    for (i=0; i<n; i++){
        if (i) printf(" ");
        if (i==x)
            printf("[%ld]", A[i]);
        else
            printf("%ld", A[i]);
    }
    printf("\n");
    free(A);
}
//============================================================================
#ifndef DESKTOP
int main()
{
    input(stdin);
    return 0;
}
#endif