297. Serialize and Deserialize Binary Tree

• deque inherits from linkedlist(collection).
• represent separator and null as a String.
• string and char are not same.
• append complete subtree ex “1,null,null,”

105. Construct Binary Tree from Preorder and Inorder Traversal

• recursive solution is straightforward. The only thing is that jump idx to find right start in inorder array. To speed up, we can cache inorder and index in the front.
• iterative not understand