230. Kth Smallest Element in a BST ——迭代本质：a=xx1 while some_condition: a=xx2

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def kthSmallest(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: int
        """
        # left first
        node = root
        nodes = []
        n = 0
        while node:
            nodes.append(node)
            node = node.left        
        while nodes:
            node = nodes.pop() # min val
            n += 1
            if n == k:
                return node.val
            node = node.right
            while node:
                nodes.append(node)
                node = node.left
        return None