中序遍历

利用中序遍历找出第k个不为空的结点即可

java 代码

class Solution {
    private List<TreeNode> list = new ArrayList<>();  //存放中序遍历的结点
    public TreeNode kthNode(TreeNode root, int k) {
        helper(root);
        return list.get(k - 1);
    }
    public void helper(TreeNode root) {
        if (root.left == null) list.add(root);
        else {
            helper(root.left);
            list.add(root);
        }
        if (root.right != null) helper(root.right);
    }
}

java 代码

class Solution2 {
    private int count = 0;
    public TreeNode kthNode(TreeNode root, int k) {
        if (root == null) return null;
        TreeNode node = kthNode(root.left, k);
        if (node != null) return node;
        count++;
        if (count == k) return root;
        node = kthNode(root.right, k);
        if (node != null) return node;
        return null;
    }
}