Tree traversal

Here is a summary of a few common implementations of tree traversals.

In order traversal

# recursive
def dfs(root):
  if not root:
    return
  dfs(root.left)
  visit(root)
  dfs(root.right)
  return

# iterative
def inorder(root):
  stack = []
  node = root
  while node or stack:
    if node:
      stack.append(node)
      node = node.left
    else:
      node = stack.pop()
      visit(node)
      node = node.right

pre order

# recursive
def dfs(root):
  if not root:
    return
  visit(root)
  dfs(root.left)
  dfs(root.right)

# iterative 1
def preorder(root):
  stack = [root]
  while stack:
    node = stack.pop()
    visit(node)
    if node.right:
      stack.append(node.right)
    if node.left:
      stack.append(node.left)

# iterative 2
def preorder(root):
  stack = []
  node = root
  while node or stack:
    if node:
      stack.append(node)
      visit(node)
      node = node.left
    else:
      node = stack.pop()
      node = node.right

post order

# recursive
def dfs(root):
  if not root:
    return
  dfs(root.left)
  dfs(root.right)
  visit(root)
  return

# iterative only visit a node when we know its right child (if exist) has been visited before.
def postorder(root):
  last = None
  stack = []
  node = root
  while node or stack:
    if node:
      stack.append(node)
      node = node.left
    else:
      top = stack[-1]
      # its right node has not been visited
      if top.right and top.right != last:
        node = top.right
        stack.append(node)
      else:
        last = top
        visit(top)

# post order = reverse of (pre order (root, right, left))
# code ommitted as an exercise. See discussion 1 for answer.

Reference

discussion 1
discussion 2