leetcode230

/**
 * 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);
        }
}

https://leetcode.com/problems/kth-smallest-element-in-a-bst/#/description

补充一个python的实现，使用二叉树的中序遍历：

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