spiral BT python

# http://code2begin.blogspot.com
# Program to print nodes of tree using spiral order traversal
# node class
class node:
    def __init__(self, element):
        self.data = element
        self.left = None
        self.right = None


# function to print the tree using spiral level wise traversal
def spiral_traversal(root):
    if root is None:
        return

    STACK1 = list()
    STACK2 = list()
    level = 1
    STACK1.append(root)
    while len(STACK1) > 0 or len(STACK2) > 0:
        print("\nNodes at level " + str(level) + " are : ")
        level += 1
        while len(STACK1) > 0:
            # pop the first node from the queue
            NODE = STACK1[0]
            del STACK1[0]
            print(NODE.data, end=" ")

            # push the left child on queue
            if NODE.right is not None:
                STACK2.append(NODE.right)

            # push the right child on queue
            if NODE.left is not None:
                STACK2.append(NODE.left)

        print("\nNodes at level " + str(level) + " are : ")
        level += 1
        while len(STACK2) > 0:
            # pop the first node from the queue
            NODE = STACK2[0]
            del STACK2[0]
            print(NODE.data, end=" ")

            # push the left child on queue
            if NODE.right is not None:
                STACK1.append(NODE.right)

            # push the right child on queue
            if NODE.left is not None:
                STACK1.append(NODE.left)



head = node(1)
head.left = node(2)
head.right = node(3)
head.left.left = node(4)
head.left.right = node(5)
head.right.right = node(6)
head.left.left.right = node(7)
head.right.right.left = node(8)
head.left.left.right.left = node(9)
head.left.left.right.left.left = node(10)
head.right.right.left.right = node(11)
print("\n\nSPIRAL Level wise traversal of the above binary tree is : ")
spiral_traversal(head)