230. Kth Smallest Element in a BST

class Solution:
    def kthSmallest(self, root, k):
        """
        :type root: TreeNode
        :type k: int
        :rtype: int
        """
        if not root:
            return 0

        ls = []
        node = root

        st = []
        while st or node:
            if node:
                st.append(node)
                node = node.left
            else:
                node = st.pop()
                ls.append(node.val)
                node = node.right

        return ls[k-1]