😳1110 Delete nodes and return forest

https://leetcode.com/problems/delete-nodes-and-return-forest/

重点在于递归函数的返回值以及传入的参数rootDeleted，返回值表示当前node有没有被删掉，这是告诉parent要不要切断edge，也就是把child设为null，rootDeleted用来决定当前的node是不是新的root，只有它没被删掉而且它的parent都被删掉了它才会是新的root

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
        Set<Integer> toDelete = new HashSet<>();
        for (int d : to_delete) toDelete.add(d);
        List<TreeNode> ans = new ArrayList<>();
        del(root, ans, toDelete, true);
        return ans;
    }
    // return true if this root is deleted
    private boolean del(TreeNode root, List<TreeNode> list, Set<Integer> toDel, boolean rootDeleted) {
        if (root == null) return false;
        if (toDel.contains(root.val)) {
            del(root.left, list, toDel, true);
            del(root.right, list, toDel, true);
            return true;
        } else {
            if (del(root.left, list, toDel, false)) {
                root.left = null;
            }
            if (del(root.right, list, toDel, false)) {
                root.right = null;
            }
            if (rootDeleted) list.add(root);
            return false;
        }
    }
}