LeetCode 156：上下翻转二叉树

题目

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.

For example: Given a binary tree {1,2,3,4,5},

    1
   / \
  2   3
 / \
4   5

return the root of the binary tree [4,5,2,#,#,3,1].

   4
  / \
 5   2
    / \
   3   1

解答

右结点要么为空，要么是有左兄弟的叶结点。把题目中的树拆成一个个小三角形(父结点、左子结点、右子结点)来分别翻转

方法一：迭代

先把

    1
   / \
  2   3

 变成

    2
   / \
  3   1

然后把

  2
 / \
4   5