给定一个二叉树，原地将它展开为一个单链表。
例如，给定二叉树
1
/
2 5
/ \
3 4 6
将其展开为：
1
2
3
4
5
6
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
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# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def flatten(self, root: TreeNode) -> None:
"""
Do not return anything, modify root in-place instead.
"""
while root:
if root.left:
temp = root.left
while temp.right:
temp = temp.right //遍历到左子树最右
temp.right = root.right
root.right = root.left
root.left = None
root = root.right
