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 main() {
    let n = read::<usize>();
    let mut a = read_vec::<i32>();

    let num = partition(&mut a, 0, n);
    print_vec_part(&a, num as usize);
}

fn partition(a: &mut [i32], p: i32, r: usize) -> i32 {
    let x = a[r - 1];

    let mut i = p - 1;
    for j in p as usize..r {
        if a[j] <= x {
            i += 1;
            a.swap(i as usize, j);
        }
    }
    i
}


fn print_vec_part<T: std::fmt::Display>(v: &Vec<T>, num: usize) {
    for a in &v[0..num] {
        print!("{} ", a);
    }
    print!("[{}] ", v[num]);
    for a in &v[num + 1..v.len() - 1] {
        print!("{} ", a);
    }
    println!("{}", v[v.len() - 1]);
}

/// 標準入力から一行を読み取り、指定の型に変換する関数
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line)
        .ok()
        .expect("Faild to read Vec");
    line.trim().split_whitespace()
        .map(|i| i.parse().ok().unwrap()).collect()
}