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 read_lines() -> Vec<String> {
    use std::io::Read;

    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    return buf.lines().map(|e| e.to_string()).collect::<Vec<_>>();
}

fn main() {
    let input = read_lines();
    let n: usize = input[0].parse().unwrap();
    let mut series = input[1].split_whitespace().map(|s| s.parse().unwrap()).collect::<Vec<u32>>();
    let r = n - 1;

    let q = partition(&mut series, 0, r);

    let small = series[0..q].iter().map(|e| format!("{}", e)).collect::<Vec<String>>().join(" ");
    let large = series[q+1..r+1].iter().map(|e| format!("{}", e)).collect::<Vec<String>>().join(" ");
    println!("{} [{}] {}", small, series[q], large);
}

fn partition<T: Ord + Clone>(series: &mut Vec<T>, p: usize, r: usize) -> usize {
    let x = series[r].clone();
    let mut i = (p as i64) - 1;
    for j in p..r {
        if series[j] <= x {
            swap(series, (i + 1) as usize, j);
            i += 1;
        }
    }
    swap(series, (i + 1) as usize, r);
    return (i + 1) as usize;
}

fn swap<T: Clone>(series: &mut Vec<T>, i: usize, j: usize) {
    let tmp = series[i].clone();
    series[i] = series[j].clone();
    series[j] = tmp;
}