Acwing786. The kth number

Given a length of  n  The whole number sequence of , And an integer  k, Please use the quick selection algorithm to find out the number sequence from small to large  k  Number .

Input format

The first line contains two integers  n  and  k.

The second line contains  n  It's an integer （ All integers are in  1∼10^9 Within the scope of ）, Represents an integer sequence .

Output format

Output an integer , It represents the second part of a sequence  k  decimal .

Data range

1≤n≤100000,
1≤k≤n

sample input ：

5 3
2 4 1 5 3

sample output ：

3

It's the fast platoon , Focus on mastering the core of fast scheduling algorithm , The core idea of partition

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 100010;

int q[N];

void quick_sort(int q[], int l, int r)
{
    if (l >= r) return; 

    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while (i < j)
    {
        do {
            i ++;
        }while (q[i] < x);
        do{
            j -- ;
            
        }while (q[j] > x);
        if (i < j) swap(q[i], q[j]);
    }

    quick_sort(q, l, j);
    quick_sort(q, j + 1, r);
}

int main()
{
    int n , k;
    scanf("%d%d", &n , &k);

    for (int i = 0; i < n; i ++ ) scanf("%d", &q[i]);

    quick_sort(q, 0, n - 1);

    for (int i = 0; i < n; i ++ ) if(i == k - 1)printf("%d ", q[i]);

    return 0;
}