Given a binary tree, return the inorder, preorder, postorder traversal of its nodes' values.
Example:
Input: [1,null,2,3] 1 \ 2 / 3 inorder Output: [1,3,2]
preorder Output: [1,2,3]
postorder Output: [3,2,1]
这三个题目的思路及总结都在LeetCode questions conlusion_InOrder, PreOrder, PostOrder traversal里面.