05_binary_tree_traversal_reverse_levelorder.py

class Stack(object):
    def __init__(self):
        self.items = []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        if not self.is_empty():
            return self.items.pop()
        
    def is_empty(self):
        return len(self.items) == 0
    
    def peek(self):
        if not self.is_empty():
            return self.items[-1]
        
    def size(self):
        return len(self.items)
    
    def __len__(self):
        return self.size()
    


class Queue(object):
    def __init__(self):
        self.items = []

    def enqueue(self, item):
        self.items.insert(0, item)

    def dequeue(self):
        if not self.is_empty():
            return self.items.pop()

    def is_empty(self):
        return len(self.items) == 0

    def peek(self):
        if not self.is_empty():
            return self.items[-1].value

    def __len__(self):
        return self.size()

    def size(self):
        return len(self.items)
    

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

class BinaryTree(object):
    def __init__(self, root):
        self.root = Node(root)

    def print_tree(self, traversal_type):
        
        if traversal_type == "levelorder":
            return self.levelorder_print(self.root)

        elif traversal_type == "reverse levelorder":
            return self.reverse_levelorder_print(self.root)
        
        else:
            print("Traversal type " + str(traversal_type) + " is not supported.")
            return False
    
    def levelorder_print(self, start):
        if start is None:
            return 

        queue = Queue()
        queue.enqueue(start)

        traversal = ""
        while len(queue) > 0:
            traversal += str(queue.peek()) + "-"
            node = queue.dequeue()

            if node.left:
                queue.enqueue(node.left)
            if node.right:
                queue.enqueue(node.right)

        return traversal
    
    def reverse_levelorder_print(self, start):
        if start is None:
            return
        
        queue = Queue()
        stack = Stack()
        queue.enqueue(start)

        traversal = ""
        while len(queue) > 0:
            node = queue.dequeue()
            stack.push(node)

            if node.right:
                queue.enqueue(node.right)
            if node.left:
                queue.enqueue(node.left)

        while len(stack) > 0:
            traversal += str(stack.peek().value) + "-"
            stack.pop()

        return traversal

        


    
'''
    Reverse Level-Order Traversal
    
                    1
                  /   \
                 2      3
                /  \          
               4    5     

    1-2-3-4-5-
'''

        

if __name__ == "__main__":

    tree = BinaryTree(1)
    tree.root.left = Node(2)
    tree.root.right = Node(3)
    tree.root.left.left = Node(4)
    tree.root.left.right = Node(5)

    X = tree.print_tree("reverse levelorder")
    print(X)