The number of nodes in the given tree is at most 1000.
Each node has a distinct value between 1 and 1000.
to_delete.length <= 1000
to_delete contains distinct values between 1 and 1000.
Solutions
Solution 1: DFS
First, we use a hash table or an array of length 1001, s, to record all nodes that need to be deleted.
Next, we design a function dfs(root) that returns the root of the subtree with root as the root after deleting all nodes that need to be deleted. The execution logic of the function dfs(root) is as follows:
If root is null, we return null;
Otherwise, we recursively execute dfs(root.left) and dfs(root.right), and assign the return values to root.left and root.right respectively. If root does not need to be deleted, we return root; if root needs to be deleted, we check whether root.left and root.right are null. If they are not null, we add them to the answer array; finally, we return null.
In the main function, we call dfs(root). If the result is not null, it means that the root node does not need to be deleted, and we add the root node to the answer array. Finally, we return the answer array.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the number of nodes in the tree.