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 Rust (judge verdict: Accepted)

fn print_vec(arr: &Vec<i32>, i_threshold: usize) {
    for i in 0..arr.len() {
        if i == i_threshold {
            print!("[{}]", arr[i]);
        } else {
            print!("{}", arr[i]);
        }
        if i < arr.len() - 1 {
            print!(" ");
        }
    }
}

fn partition(inputs: &mut Vec<i32>, left: i32, right: i32) -> usize {
    let mut i = (left - 1) as usize;
    let left = left as usize;
    let right = right as usize;

    let threshold = inputs[right - 1];

    for j in left..(right - 1) {
        if inputs[j] <= threshold {
            i = (i as i32 + 1) as usize;

            let tmp = inputs[j];
            inputs[j] = inputs[i];
            inputs[i] = tmp;
        }
    }
    {
        let tmp = inputs[i + 1];
        inputs[i + 1] = inputs[right - 1];
        inputs[right - 1] = tmp;
    }
    return i + 1;
}

fn main() {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).ok();
    let n = line.trim().parse::<i32>().unwrap();

    let mut line = String::new();
    std::io::stdin().read_line(&mut line).ok();
    let mut inputs: Vec<i32> = line.split_whitespace()
        .map(|e| e.parse::<i32>().ok().unwrap())
        .collect();

    let (left, right) = (0, n);
    let i_threshold = partition(&mut inputs, left, right);

    print_vec(&inputs, i_threshold);
    println!();
}