In Postorder traversal, the processing order is left-right-current. So we need to visit the left section first before visiting other parts. We will try to traverse down to the tree as left as possible for each node of the tree. For each current node, if the right child is present then push it into the stack before pushing the current node while root is not NULL/None. Now pop a node from the stack and check whether the right child of that node exists or not. If it exists, then check whether it's same as the top element or not. If they are same then it indicates that we are not done with right part yet, so before processing the current node we have to process the right part and for that pop the top element(right child) and push the current node back into the stack. At each time our head is the popped element. If the current element is not the same as the top and head is not NULL then we are done with both the left and right section so now we can process the current node. We have to repeat the previous steps until the stack is empty.
def Postorder_iterative(head):
if head is None:
return None
sta=stack()
while True:
while head is not None:
if head.r:
sta.push(head.r)
sta.push(head)
head=head.l
if sta.top is -1:
break
head = sta.pop()
if head.r is not None and sta.top is not -1 and head.r is sta.A[sta.top]:
x=sta.pop()
sta.push(head)
head=x
else:
print(head.val,end = ' ')
head=None
print()