230 Kth Smallest Element in a BST

这道题就是数点 divide and conquer

class Solution:
    # @param {TreeNode} root
    # @param {integer} k
    # @return {integer}
    def kthSmallest(self, root, k):
        n = self.countNodes(root.left)
        if k == n + 1:
            return root.val
        elif k < n + 1:
            return self.kthSmallest(root.left, k)
        else:
            return self.kthSmallest(root.right, k - n - 1)

    def countNodes(self, root):
        if root == None:
            return 0
        return 1 + self.countNodes(root.left) + self.countNodes(root.right)