二叉树先序遍历-非递归版

      public static void  preOrderPrint(Node head){
        System.out.print("pre-order: ");
        if (head!=null){
            Stack<Node> stack=new Stack<>();
            stack.push(head);
            while (!stack.isEmpty()){
                head = stack.pop();
                System.out.print(head.value+" ");
                if (head.right!=null){
                    stack.push(head.right);
                }
                if (head.left!=null){
                    stack.push(head.left);
                }
            }
        }
    }