[LC] 1110. Delete Nodes And Return Forest

Given the root of a binary tree, each node in the tree has a distinct value.

After deleting all nodes with a value in to_delete, we are left with a forest (a disjoint union of trees).

Return the roots of the trees in the remaining forest.  You may return the result in any order.

Example 1:

Input: root = [1,2,3,4,5,6,7], to_delete = [3,5]
Output: [[1,2,null,4],[6],[7]]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
        List<TreeNode> res = new ArrayList<>();
        Set<Integer> set = new HashSet<>();
        for(int num : to_delete) {
            set.add(num);
        }
        if (!set.contains(root.val)) {
            res.add(root);
        }
        helper(root, res, set);
        return res;
    }
    
    private TreeNode helper(TreeNode root, List<TreeNode> res, Set<Integer> set) {
        if (root == null) {
            return null;
        }
        root.left = helper(root.left, res, set);
        root.right = helper(root.right, res, set);
        if (set.contains(root.val)) {
            if (root.left != null) {
                res.add(root.left);
            }
            if (root.right != null) {
                res.add(root.right);
            }
            return null;
        }
        return root;
    }
}