104. maximum depth of binary tree

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class (object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root == None:
            return 0
        myStack = []
        node = root  # working pointer
        treeDepth = 0
        maxDepth = 0
        
        while node != None or myStack != []:
            while node != None:  # if still have left child
                treeDepth += 1
                myStack.append((node, treeDepth))
                node = node.left
            # run out of lefts
            now = myStack.pop()
            node = now[0]
            maxDepth = max(maxDepth, treeDepth)
            treeDepth = now[1]
            node = node.right
        return maxDepth