DFS Iterative Binary Tree Traversal

dfs_iterative_binary_tree_traversal.py

# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right


def iterative_pre_order_traversal(root: TreeNode):
  stack = [root]
  while stack:
    node = stack.pop()
    # Here we process the node before the left and right children
    print(node.val)
    # We append the right node first because stack is LIFO
    if node.right:
      stack.append(node.right)
    if node.left:
      stack.append(node.left)


def iterative_in_order_traversal(root: TreeNode):
  stack = []
  node = root
  
  while stack or node:
    # Navigate to the left-most node first
    while node:
      stack.append(node)
      node = node.left
    
    node = stack.pop()
    # Notice we process the left sub-tree
    # before processing the current node and
    # moving to the right sub-tree
    print(node.val)
    node = node.right
      
      
def iterative_post_order_traversal(root: TreeNode):
  stack = []
  last_visited = None
  node = root
  while stack or node:
    # Traverse the left subtree if not visited already
    if node:
      stack.append(node)
      node = node.left
    # Traverse the right subtree if not visited already
    elif stack[-1].right and stack[-1].right != last_visited:
      node = stack[-1].right
    else:
      # Process the node
      print(stack[-1].val)
      last_visited = stack.pop()