• inorder(node.left)
  • visit node
  • inorder(node.right)

It is easy to write a recursive version.

Convert to an iterative version

There are a lot of articles about how to do this using a stack, but, I have no enough memory to understand why they are using stack like that. As a result, I am going to write a function call simulating verison.

What happens when call to a function and return from a function


  • save all local vars
  • save current code address (continue executing after callee returned)
  • put params somewhere
  • jump to the function


  • retore all local vars
  • jump to saved caller code address

Simulating address

switch is a good idea.

(function entrance)

  // return or new call start

  take params from somewhere
  take saved address from somewhere

  switch saved address
    Line 1:

      do something

    Line 100:

      do someting

      // making call
      save some var
      save next address 250

      put some param for new call
      jump to (function entrance)

    Line 250:

      do something

Back to the problem

  • Use a stack to save state, a state contains a param TreeNode and a return address.
  • Use a loop to execute all state (simulating recursive function call)
  • whenever there need a new call to function
    • push current state to stack
    • create and push a new state with param to stack
    • jump to loop begin
while stack is not empty

   current = stack.pop()

   switch current.returnAddress

      stack.push current

      new state { returnAddress = IN, param = current.left }
      stack.push new state

      visit current.param


      stack.push current

      new state { returnAddress = DONOTHING, param = current.right }
      stack.push new state