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)

use std::io::*;
use std::str::FromStr;

//https://qiita.com/tubo28/items/e6076e9040da57368845
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

//a:対象配列 p:分割する左端のインデックス r:分割する右端のインデックス、基準値のインデックス
fn partition(a:&mut Vec<u32>, p:usize, r:usize) -> usize{
    //基準値設定
    let x = a[r];
    //左側に分割するインデックス
    let mut i = p;
    //左端から右端までを基準値と比較し、基準値以下であれば左側に移動する
    //このとき基準値以上の値が右側に移動する
    for j in p..r{
        if a[j] <= x {
            //基準値以下と基準値より大きい値をswap
            a.swap(i,j);
            i = i + 1;
        }
    }
    //基準値を境界に挿入する
    a.swap(i,r);
    i
}

fn main(){
    let n: usize = read();
    let mut a: Vec<u32> = (0..n).map(|_| read()).collect();

    let p = partition(&mut a, 0, n-1);
    for i in 0..a.len(){
        if i == p {
            print!("[{}]",a[i]);
        }else{
            print!("{}",a[i]);
        }
        if i != a.len() - 1{
            print!(" ");
        }else{
            println!("");
        }
    }

}