Help me understand Inorder Traversal without using recursion
- by vito
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 haven't understood the inner working of recursion.
This is what I've tried so far:
def traverseInorder(node):
lifo = Lifo()
lifo.push(node)
while True:
if node is None:
break
if node.left is not None:
lifo.push(node.left)
node = node.left
continue
prev = node
while True:
if node is None:
break
print node.value
prev = node
node = lifo.pop()
node = prev
if node.right is not None:
lifo.push(node.right)
node = node.right
else:
break
The inner while-loop just doesn't feel right. Also, some of the elements are getting printed twice; may be I can solve this by checking if that node has been printed before, but that requires another variable, which, again, doesn't feel right. Where am I going wrong?
I haven't tried postorder traversal, but I guess it's similar and I will face the same conceptual blockage there, too.
Thanks for your time!
P.S.: Definitions of Lifo and Node:
class Node:
def __init__(self, value, left=None, right=None):
self.value = value
self.left = left
self.right = right
class Lifo:
def __init__(self):
self.lifo = ()
def push(self, data):
self.lifo = (data, self.lifo)
def pop(self):
if len(self.lifo) == 0:
return None
ret, self.lifo = self.lifo
return ret
