leetcode: 144. Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?

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


# DFS
class Solution():
    def preorderTraversal(self, root):
        res = []
        self.preorder(root, res)
        return res
    def preorder(self, root, res):
        if root:
            res.append(root.val)
            self.preorder(root.left, res)
            self.preorder(root.right, res)


# Stack
class Solution():
    def preorderTraversal(self, root):
        res, stack = [], [(root, False)]
        while stack:
            root, visited = stack.pop()
            if not root:
                continue
            if visited:
                res.append(root.val)
            else:
                stack.append((root.right, False))
                stack.append((root.left, False))
                stack.append((root, True))
        return res