Help me understand Inorder Traversal without using recursion

Question

I am able to understand preorder traversal without using recursion, but I\'m having a hard time with inorder traversal. I just don\'t seem to get it, perhaps, because I have

Answer 1

Simple iterative inorder traversal without recursion

'''iterative inorder traversal, O(n) time & O(n) space '''

class Node:
    def __init__(self, value, left = None, right = None):
        self.value = value
        self.left = left
        self.right = right

def inorder_iter(root):

    stack = [root]
    current = root

    while len(stack) > 0:
        if current:
            while current.left:
                stack.append(current.left)
                current = current.left
        popped_node = stack.pop()
        current = None
        if popped_node:
            print popped_node.value
            current = popped_node.right
            stack.append(current)

a = Node('a')
b = Node('b')
c = Node('c')
d = Node('d')

b.right = d
a.left = b
a.right = c

inorder_iter(a)