从预遍历列表Python构造完整的二叉树

Question

How do I construct a binary tree (not binary search tree) from a preorder listing in Python using only the built in list? Each node in the preorder listing also has a "flag" that shows if it's a leaf or internal node. And each node has 2 children (No 0 or 1 child)

I made a class to represent a Node

class Node:
    def __init__(self, initType):
        self.type = initType
        self.leftChild = None
        self.rightChild = None

I think I should use a stack here, but I don't want to import the stack library.

Solutions I found online are for BST only or the input consists of an inorder list and a preorder list, not just the preorder list alone.

Answer 1

You can not uniquely re-create a binary tree given only the pre order traversal. That is why the solutions you found online were for a BST or also provided the in order list as well.

For example, if the pre order list was gives as [1, 2, 3]

then your original binary tree could be:

         1
       /
     2
   /
  3

or it could be:

1
  \
   2
    \
     3 

** notice how they are different trees but would have the same pre order traversal lists!

If this question was asked in an interview, it was meant to see if you would notice the fact that a unique binary tree could not be implemented and have you clarify the question: "Is the binary tree a binary search tree?"

It is very common in interviews to leave out facts from a question to have you ask questions.