大佬们为啥我一个WA，两个TLE啊

#include<bits/stdc++.h>
using namespace std;

long long a[6000000],b[6000000];
long long n,k;
void msort(long long arr[],long long brr[],long long l,long long r)
{
	if (l >= r) return;
	long long mid = (l + r) / 2;
	msort(arr,brr,l,mid),msort(arr,brr,mid + 1,r);
	long long i = l,j = mid + 1,k = l;
	while (i <= mid && j <= r)
	{
		if (arr[i] <= arr[j])
		{
			brr[k] = arr[i];
			k++,i++;
		}
		else
		{
			brr[k] = arr[j];
			k++,j++;
		}
	}
	
	while (i <= mid)
	{
		brr[k] = arr[i];
		k++,i++;
	}
	while (j <= r)
	{
		brr[k] = arr[j];
		k++,j++;
	}
	for (int i = l;i <= r;i++)
	{
		arr[i] = brr[i];
	}
}

int main()
{
	cin >> n >> k;
	for (int i = 1;i <= n;i++) {
		cin >> a[i];
	}
	msort(a,b,1,n);
	cout << a[k];
	return 0;
}

------------