2792. Count Nodes That Are Great Enough

For each node, calculate a list of k values representing k smallest values in the subtree of that node.

To check if a node is great enough, get the described list in the first hint for its children and merge them. Since the resulting list may contain more than k elements, pick k smallest values and discard the extra ones.

Now check if the merged list has exactly k elements, and the current node's value is greater than the greatest element in the list, then that node is great enough.