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

解题思路：
使用栈 list，申请一个指针 point 修改 root，通过出栈顺序移动指针（point = top）。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: None Do not return anything, modify root in-place instead.
        """
        # pointer = TreeNode(None)
        pointer = root
        stack = [root]
        # print(root)
        # print(stack)
        while stack: 
            # print(stack)
            top = stack.pop()
            print("top----",top)
            if not top:
                continue
            stack.append(top.right)
            stack.append(top.left)
            pointer.right = top
            pointer.left = None
            # print("root---",root)
            print("pointer----",pointer)
            pointer = top
            print("root---",root)
        # print("root---",root)
        # print(dummy)