Find the th largest number of numbers. (Use divide-and-conquer method)

Find the i th largest number of numbers. (Use divide-and-conquer method)

#include <iostream>
using namespace std;
const int N = 1e6 + 10;
int a[N];
int ans[N];
void mergeSort(int a[], int l, int r)
{
	if (l >=r) return;
	int mid = (l + r) / 2;
	mergeSort(a, l, mid );
	mergeSort(a, mid + 1, r);
	int k1=0, i = l, j = mid+1;
	while (i <=mid&& j<=r)
	{
		if (a[i] >=a[j])
			ans[k1++] = a[i++];
		else
			ans[k1++] = a[j++];
	}
	while(i<=mid)
		ans[k1++] = a[i++];
	while (j <= r)
		ans[k1++] = a[j++];
	for (int i = l, j=0; i <=r; i++,j++)
	{
		a[i] = ans[j];
	}
}
int partition(int a[], int l, int r, int x)
{
	int i = l - 1,  j = r + 1;
	while (i < j)
	{
		do i++;
		while (a[i] > x);
		do j--;
		while (a[j] < x);
		if (i < j)swap(a[i], a[j]);
	}
	return i;
}
int findK(int a[], int l, int r, int k)
{
	if (r - l < 75)
		return a[l + k - 1];
	for (int i = 0; i < (r - l - 4) / 5; i += 5)
		mergeSort(a, i, i + 4);
	for (int i = 0; i < (r - l - 4) / 5; i++)
		swap(a[i+l], a[l + 5 * i + 2]);
	int x = findK(a, l, (r - l - 4) / 5, (r - l - 4) / 10);
	int i = partition(a, l, r, x);
	int j = i - l + 1;
	if (k >= j)
		findK(a, i+1, r, k - j);
	else
		findK(a, l, j, k);
}

int main()
{
	int n;
	cin >> n;
	int k;
	cout << "please enter the k to find the kth in the array" << endl;
	cin >> k;
	for (int i = 0; i < n; i++)
		a[i]=rand();

	int i = findK(a, 0, n-1, k);
	cout << "The kth number is" << i << endl;
	return 0;
}

//* 1.首先将数组分为五五一组，并且进行排序
//* 2.将每个组a[2]也就是中位数，挪到a[0]-a[(r-p-4)/5],也就是数组前几个，（r-p-4)/5代表五个组的个数
//* 3.递归找到中位数的中位数
//* 4.对整个数组进行排序，使中位数的中位数左边<中位数的中位数，右边>中位数的中位数,参考快排
//* 5.定位中位数的中位数的索引，分情况递归。