https://leetcode.com/problems/binary-tree-inorder-traversal/
Given the root of a binary tree, return the inorder traversal of its nodes' values.
Example 1:
Input: root = [1,null,2,3] Output: [1,3,2] Example 2:
Input: root = [] Output: [] Example 3:
Input: root = [1] Output: [1] Example 4:
Input: root = [1,2] Output: [2,1] Example 5:
Input: root = [1,null,2] Output: [1,2]
Constraints:
The number of nodes in the tree is in the range [0, 100]. -100 <= Node.val <= 100
Follow up:
Recursive solution is trivial, could you do it iteratively?