O(n) time and O(1) space.

Author: 149ps

156. Binary Tree Upside Down/156. Binary Tree Upside Down.py

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+"""
+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.
+
+Example:
+
+Input: [1,2,3,4,5]
+
+    1
+   / \
+  2   3
+ / \
+4   5
+
+Output: return the root of the binary tree [4,5,2,#,#,3,1]
+
+   4
+  / \
+ 5   2
+    / \
+   3   1
+Clarification:
+
+Confused what [4,5,2,#,#,3,1] means? Read more below on how binary tree is serialized on OJ.
+
+The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
+
+Here's an example:
+
+   1
+  / \
+ 2   3
+    /
+   4
+    \
+     5
+The above binary tree is serialized as [1,2,3,#,#,4,#,#,5].
+"""