practice makes perfect
- 94 暖身 Binary Tree Inorder Traversal
- 98 isBST https://www.youtube.com/watch?v=MILxfAbIhrE 用遞迴解
- 173 https://leetcode.com/problems/binary-search-tree-iterator/
- 230 https://leetcode.com/problems/kth-smallest-element-in-a-bst/
- 783 https://leetcode.com/problems/minimum-distance-between-bst-nodes/
- 426 BST to doubly linked list (should get familiar with 94 first)