vertical order bt python

# http://code2begin.blogspot.com
# Program to print the nodes of the binary tree in vertical order using horizontal distance from the root node

# node class
class node:
    def __init__(self, element):
        self.data = element
        self.hd = -1
        self.left = None
        self.right = None



# function to print the nodes of the binary tree in vertical order
# using horizontal distance of a node from the root node
def vertical_order(root):
    if root is None:
        return

    # initialize a map and queue
    # we will use iterative BFS traversal to solve this
    M = dict()
    Q = list()

    # push root in queue and initialize horizontal distance
    Q.append(root)
    horizontal_distance = root.hd

    while len(Q) > 0:
        current = Q[0]
        del Q[0]
        if current.hd in M.keys():
            M[current.hd].append(current.data)
        else:
            M[current.hd] = [current.data]

        # update horizontal distance of the left child and put it in the queue
        if current.left is not None:
            current.left.hd = current.hd - 1
            Q.append(current.left)

        # update horizontal distance of the right child and put it in the queue
        if current.right is not None:
            current.right.hd = current.hd + 1
            Q.append(current.right)

    # printing the contents of the map
    for horizontal_distance, elements in zip(M.keys(), M.values()):
        print("Nodes at " + str(horizontal_distance) + " : " + str(elements))



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)
head.hd = 0

print("Vertical order traversal of the binary tree is : ")
vertical_order(head)