114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

First

根據(jù)棧的思想，通過深度優(yōu)先遍歷將所有的節(jié)點(diǎn)退出棧中，再由棧彈出每個節(jié)點(diǎn)進(jìn)行拼接

from quixote.structs.tree import TreeNode

class Solution(object):
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: None Do not return anything, modify root in-place instead.
        """
        self.stack = []
        self.dfs(root)
        print(self.stack)
        tmp = None
        while self.stack:
            node = self.stack.pop()
            node.right = tmp
            tmp = node
        return tmp

    def dfs(self, root):
        self.stack.append(root)
        if root.left:
            self.dfs(root.left)
        if root.right:
            self.dfs(root.right)