leetcode: 94. Binary Tree Inorder Traversal

# Given a binary tree, return the inorder traversal of its nodes' values.
#
# For example:
# Given binary tree [1,null,2,3],
#
#    1
#     \
#      2
#     /
#    3
#
# return [1,3,2].
#
# Note: Recursive solution is trivial, could you do it iteratively?

class Solution():
    def inorderTraversal(self, root):
        res = []
        self.inorder(root, res)
        return res
    def inorder(self, root, res):
        if not root:
            return
        self.inorder(root.left, res)
        res.append(root.val)
        self.inorder(root.right, res)

class Solution():
    def inorderTraversal(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, True))
                stack.append((root.left, False))
        return res