/** * Definition for a binary tree node. * public class TreeNode { *     public int val; *     public TreeNode left; *     public TreeNode right; *     public TreeNode(int x) { val = x; } * } */public class Solution {    public int KthSmallest(TreeNode root, int k)        {            int count = countNodes(root.left);            if (k <= count)            {                return KthSmallest(root.left, k);            }            else if (k > count + 1)            {                return KthSmallest(root.right, k - 1 - count); // 1 is counted as current node            }            return root.val;        }        public int countNodes(TreeNode n)        {            if (n == null) return 0;            return 1 + countNodes(n.left) + countNodes(n.right);        }}

 

补充一个python的实现：

1 class Solution: 2     def __init__(self): 3         self.l = list() 4  5     def preOrder(self,root): 6         if root != None: 7             self.preOrder(root.left) 8             self.l.append(root.val) 9             self.preOrder(root.right)10 11     def kthSmallest(self, root: TreeNode, k: int) -> int:12         self.preOrder(root)13         return self.l[k-1]