375. Clone Binary Tree
For the given binary tree, return a deep copy of it.
Example
Given a binary tree:
1
/ \
2 3
/ \
4 5
return the new binary tree with same structure and same value:
1
/ \
2 3
/ \
4 5
思路：递归
1、new 一个新节点记为newRoot
2、递归左右子节点
3、return newRoot
