kth_binary_tree.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 helper(self, root: TreeNode, k: int):
        l = []
        if root.left is not None:
            l.extend(self.helper(root.left, k))

        l.append(root.val)
            
        if len(l) >= k:
            return l
        
        if root.right is not None:
            l.extend(self.helper(root.right, k))
            
        return l
        
    
    def kthSmallest(self, root: TreeNode, k: int) -> int:
        return self.helper(root, k)[k - 1]