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 Array[], int i, int j) {
    int temp;
    temp = Array[i];
    Array[i] = Array[j];
    Array[j] = temp;
}

int partition(int Array[], int pIndex, int rIndex) {
    int x = Array[rIndex];
    int i = pIndex;
    int temp;
    
    for (int j = pIndex; j < rIndex; j++)
        if (Array[j] <= x) {
            swap(Array, i, j);
            i++;
        }

    swap(Array, i, rIndex);
    return i;
}

void printArray(int array[], int n) {
    for (int i = 0; i < n; i++) {
        if (i != 0) printf(" ");
        printf("%d", array[i]);
    };
}

void printArrayHighlight(int array[], int n, int hightlightIndex) {
    for (int i = 0; i < n; i++) {
        if (i != 0) printf(" ");
        if (i == hightlightIndex)
            printf("[%d]", array[i]);
        else
            printf("%d", array[i]);
    };
}

int main(int argc, char** argv) {
    int nArray;
    scanf("%d", &nArray);
    
    int array[nArray], brrby[nArray];
    
    for (int iArray = 0; iArray < nArray; iArray++) {
        scanf("%d", &array[iArray]);
    }
    
    // do the magic
    int index = partition(array, 0, nArray - 1);
    
    printArrayHighlight(array, nArray, index);
    printf("\n");

    return (EXIT_SUCCESS);
}