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 JAVA (judge verdict: WA: Presentation Error)

import java.util.*;

public class Main {

    public static void main(String[] args) {

    	//input
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();

    	int[] A = new int[N];

        for(int i = 0; i < N; i++){
        	A[i] = sc.nextInt();
        }
        
        int result = Partition(A,0,N-1);
//        System.out.println(result);
    }

    //calculation
    public static int Partition(int[] A,int p, int r){
    	int x = A[r]; //x??????????????????A[r]???temporary??即???????????即???
    	int i = p - 1;//i????????????????????????????????????????????????????属????????????????????????????????????????
    	
//    	System.out.println("r="+r);
    	
    	for(int j = p; j<r+1; j++){
    		if(A[j]<=x){
    			i++;
    			int temp = A[i];
    			A[i] = A[j];
    			A[j] = temp;
    		} else {
    			int temp = A[i+1];
    			A[i+1] = A[r];
    			A[r] = temp;
    		}
//    		if(j == r){
//    			int temp = A[i+1];
//    			A[i+1] = A[r];
//    			A[r] = temp;
//    		}
    	}
		for(int k:A){
			if( k == x){
				System.out.print("[" + x + "] ");
				continue;
			}
			System.out.print(k + " ");
		}
//    		System.out.print("  i = " + i);
//    		System.out.print("  j = " + j);
    		System.out.println();
    		
/*    		for(int m = 0; m < r; m++){
    			if(m == i+1){
    				System.out.print("i" );
    			}else if(m == j){
    				System.out.print("j" );
    			}else {
    				System.out.print(" ");
    			}
    		}
    		System.out.println();
 */   	
    	return i;
    }
/*
    {
    //output
     for(int i = 0; i < N; i++){
    	System.out.print(Out_ar[i]);
    }
	if(i < N - 1){
		System.out.print(" ");
    	}
    }
    System.out.println();
} */
}