kth-smallest-element-in-a-bst.py

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

class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        stack = []
        while stack or root:
            while root:
                stack.append(root)
                root = root.left
            node = stack.pop()
            k -= 1
            if k == 0: return node.val
            root = node.right
        return None