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]]

Constraints:

• 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.

class Solution {
    List<TreeNode> ans;
    Set<Integer> set;

    public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
        ans = new ArrayList<>();
        set = new HashSet<>();
        for (int x : to_delete)
            set.add(x);
        helper(root);
        if (root != null && !set.contains(root.val))
            ans.add(root);
        return ans;
    }

    public TreeNode helper(TreeNode root) {
        if (root == null)
            return null;
        if (set.contains(root.val)) {
            if (root.left != null && !set.contains(root.left.val))
                ans.add(root.left);
            if (root.right != null && !set.contains(root.right.val))
                ans.add(root.right);
            helper(root.left);
            helper(root.right);
            return null;
        } else {
            root.left = helper(root.left);
            root.right = helper(root.right);
            return root;
        }
    }
}