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)

// ALDS1_6_B: Partition

fn scan<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    s.trim().parse().ok().unwrap()
}

fn scan_vec<T: std::str::FromStr>() -> Vec<T> {
    let s: String = scan();
    s.split_whitespace().map(|a| T::from_str(a).ok().unwrap()).collect()
}

fn partition(a: &mut Vec<u32>, p: usize, r: usize) -> usize {
    let x = a[r];
    let mut i = p;
    for j in p..r {
        if a[j] <= x {
            a.swap(i, j);
            i += 1;
        }
    }
    a.swap(i, r);
    i
}

fn main() {
    let n: usize = scan();
    let mut a: Vec<u32> = scan_vec();
    let q = partition(&mut a, 0, n - 1);

    for i in 0..n {
        if i > 0 {
            print!(" ");
        }
        if i == q {
            print!("[{}]", a[i]);
        } else {
            print!("{}", a[i]);
        }
    }
    println!();
}