Lazy Binary Tree Inorder Iterator

Design an iterator over a binary tree that returns nodes in inorder sequence without precomputing all values. The follow-up asks about multiple iterators and thread safety. Examples Example 1: Input: ["BSTIterator", "next", "next", "hasNext", "next", "hasNext", "next", "hasNext", "next", "hasNext"] [[[7, 3, 15, null, null, 9, 20]], [], [], [], [], [], [], [], [], []] Output: [null, 3, 7, true, 9, true, 15, true, 20, false] Constraints The number of nodes in the tree is in the range [1, 10^5]. 0 <= Node.val <= 10^6 At most 10^5 calls will be made to hasNext, and next.