new_contents: class Solution:
      # @param root, a tree node
      # @return a list of integers
      def iterative_preorder(self, root, list):
          stack = []
          while root or stack:
              if root:
                  list.append(root.val)
                  stack.append(root)
                  root = root.left
              else:
                  root = stack.pop()
                  root = root.right
          return list
      
      def recursive_preorder(self, root, list):
          if root:
              list.append(root.val)
              self.preorder(root.left,list)
              self.preorder(root.right,list)
  
      def preorderTraversal(self,root):
          list = []
          self.iterative_preorder(root,list)
          return list
  
subject: Create BinTreePreTraversal_002.py