LeetCode: Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place

我的想法是，先将左子树flatten，然后右子树flatten，然后将左子树的结果移到root.right，然后将右子树接在左子树后面。但是总是超时。

public static void flatten(TreeNode root) {
        if (root == null) return;
        TreeNode left = root.left;
        TreeNode right = root.right;
        flatten(left);
        flatten(right);
        if (left != null) {
            root.right = left;
            while(left.right != null) {
                left = left.right;
            }
            left.right = right;
        }
        
        return;
    }

 

---

 

网上看的方法是，先将左子树移到root.right，然后把右子树接在后面，最后在flatten整个右子树。。。

public void flatten(TreeNode root) {
        if(root == null){
            return;
        }
        
        if(root.left != null){
            TreeNode rightNode = root.right;
            TreeNode leftNode = root.left;
            root.left = null;
            root.right = leftNode;
            TreeNode p = leftNode;
            while(p.right != null){
                p = p.right;
            }
            p.right = rightNode;
        }
        flatten(root.right);
    }