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>
#define N 100001

int n,A[N];

int Patition(int , int);

main(){

  int i,index;

  scanf("%d",&n);

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

  index = Partition(0, n-1);
  
 for(i = 0; i < n; i++){
   if(i == index){
     printf("[%d] ",A[i]);
   } else if(i == n-1){
     printf("%d\n",A[i]);
   } else {
     printf("%d ",A[i]);
   }
 }
   return 0;
  
}

int Partition(int p, int num){

  int x,i,j,temp;

   x = A[num];
   i = p - 1;
 
  for(j = 0; j < num; j++){

     if(A[j] <= x){
       i = i + 1;
       temp = A[j];
       A[j] = A[i];
       A[i] = temp;
     }
 
  }

  temp =  A[num];
  A[num] =  A[i+1];
  A[i+1] =  temp;

  //  printf("%d\n",i+1);

  return i+1;

}