BinaryTrees | Some operations for binary tree | Dataset library

Your code starts with Node root equal to one. To the left of one is two, and to the left of two is four. Your traversal pushes two then four to the stack before the else condition is taken. Then you pop four, and since four has nothing on the right your while loop starts over. But the top of the stack is now two. To the left of two is still four, so you push four on the stack and thus your infinite loop starts.

You need a way to indicate Nodes that have already been visited. If you truly must use a stack, you should add a new attribute to your Node class such as private boolean visited and initialize it to false. After each temp = stack.pop() you would then need to set temp.visited = True, and only push onto the stack Nodes that have not been visited. Such as this: