[LeetCode]94, 144, 145 Binary Tree InOrder, PreOrder, PostOrder Traversal_Medium

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.