preorder traversal without recursion in java

Manas Bhardwaj is totally right!

But at least try to search for yourself.
Ah... you know google? Well, look what it has given to me for you: http://cybergrouch.blogspot.hu/2012/03/non-recursive-binary-tree-traversal.html[^]. But don't even dream to give that code as your homework, since your tutor will know, that it is too advanced for you. Still, you can compile and debug/trace it, and see how it works.

From how you posted your question one can assume that you have a tree to traverse where the nodes that are not leafes have some kind of operand in them and the leafes are numbers. Hoping you at least know what preorder, inorder and postorder are this is quite easy to work out using a recursive algorithm. Recursion works by presering state on the call stack where the local variables reside.

And now for something totally whack and never before thought of (kidding): Now just write an algorithm without using the implicit stack mechanism found in all the languages that support methods or procedures, but rather write a non-recursive algorithm that uses an explicit stack to keep track or where in the tree you currently are.

As the others before me already mentioned, do give it a try!

Regards,

— Manfred