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 C++14 (judge verdict: Accepted)

#include <bits/stdc++.h>
#define INF 1e9
using namespace std;

#define REPR(i,n) for(int i=(n); i >= 0; --i)
#define FOR(i, m, n) for(int i = (m); i < (n); ++i)
#define REP(i, n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define ALL(a)  (a).begin(),(a).end()
#define endl "\n"

template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
typedef long long ll;

void solve() {
    int n;
    cin >> n;
    vector<int> v(n);
    REP(i,n) cin >> v[i];

    auto partition = [&] (int p,int r) {
        auto x = v[r];
        auto i = p-1;
        FOR(j,p,r) {
            if (v[j] <= x) {
                i++;
                iter_swap(v.begin()+i,v.begin()+j);
            }
        }
        iter_swap(v.begin()+i+1,v.begin()+r);
        return i+1;
    };

    auto i = partition(0,n-1);


    REP(j,n) {
        if (j != 0) cout << " ";
        if (i == j)cout << "[";
        cout << v[j];
        if (i == j)cout << "]";
    }
    cout << endl;
}

int main() {
    solve();
    return 0;
}