So diff between binary tree and bst is bst's inorder is in ascending order (i.e. root's left is always smaller than root and root is always smaller than root.right)
This is useful in https://velog.io/@whitehousechef/Leetcode-530.-Minimum-Absolute-Difference-in-BST.
complexity
very impt, even when just store constants in local variables, if we use recursion, the space depends on the call stack.
So if balanced, log n and if skewed, it is n space.
