Iterative Preorder Traversal

Given a Binary Tree, write an iterative function to print Preorder traversal of the given binary tree.

Refer this for recursive preorder traversal of Binary Tree. To convert an inherently recursive procedures to iterative, we need an explicit stack. Following is a simple stack based iterative process to print Preorder traversal.

1) Create an empty stack nodeStack and push root node to stack. 2) Do following while nodeStack is not empty. ….a) Pop an item from stack and print it. ….b) Push right child of popped item to stack ….c) Push left child of popped item to stack

Right child is pushed before left child to make sure that left subtree is processed first.

# Python program to perform iterative preorder traversal # A binary tree node class Node: # Constructor to create a new node def __init__(self, data): self.data = data self.left = None self.right = None # An iterative process to print preorder traveral of BT def iterativePreorder(root): # Base CAse if root is None: return # create an empty stack and push root to it nodeStack = [] nodeStack.append(root) # Pop all items one by one. Do following for every popped item # a) print it # b) push its right child # c) push its left child # Note that right child is pushed first so that left # is processed first */ while(len(nodeStack) > 0): # Pop the top item from stack and print it node = nodeStack.pop() print node.data, # Push right and left children of the popped node # to stack if node.right is not None: nodeStack.append(node.right) if node.left is not None: nodeStack.append(node.left) # Driver program to test above function root = Node(10) root.left = Node(8) root.right = Node(2) root.left.left = Node(3) root.left.right = Node(5) root.right.left = Node(2) iterativePreorder(root) # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

References:

https://www.geeksforgeeks.org/iterative-preorder-traversal/

My codes

class Solution1(object): def postorderTraversal(self, root): """ :type root: TreeNode :rtype: List[int] """ if root is None: return [] stack, output = [root, ], [] while stack: root = stack.pop() output.append(root.item) if root.left is not None: stack.append(root.left) if root.right is not None: stack.append(root.right) print output return output[::-1] def preorderTraversal(self, root): """ :type root: TreeNode :rtype: List[int] """ if root is None: return [] stack, output = [root, ], [] while stack: root = stack.pop() output.append(root.item) if root.right is not None: stack.append(root.right) if root.left is not None: stack.append(root.left) return output