動機

preorder的tree threading

Problem

Given the root of a binary tree, flatten the tree into a linked list:

  • The linked list should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
  • The linked list should be in the same order as a pre-order traversal of the binary tree.

 

Example 1:

Input: root = [1,2,5,3,4,null,6]Output: [1,null,2,null,3,null,4,null,5,null,6]

Example 2:

Input: root = []Output: []

Example 3:

Input: root = [0]Output: [0]

 

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -100 <= Node.val <= 100

 

Follow up: Can you flatten the tree in-place (with O(1) extra space)?

Sol

class Solution:
    def flatten(self, root: TreeNode) -> None:
        def pre(root,prev):
            if root:
                originRight = root.right
                pro = pre(root.left, root)
                ret = pre(originRight, pro)
                if prev:
                    prev.left, prev.right = None, root
                # 目前最大的node
                if not pro and not ret:
                    return root
                elif not pro or not ret:
                    return [x for x in [pro,ret] if x][0]
                else:
                    return ret
            else:
                return None
        pre(root, None)