Inorder traversal using constant space – Morris Traversal

Usual approach using linear space (stack)
We can easily do the traversal by using a stack where we keep pushing left. If no left is there then we pop an element to print it and then push its right. This is O(n) time and O(n) space algorithm.

Binary Search Tree (BST) insert, delete, successor, predecessor, traversal, unique trees

From wiki, A binary search tree is a rooted binary tree, whose internal nodes each store a key (and optionally, an associated value) and each have two distinguished sub-trees, commonly denoted left and right. The tree additionally satisfies the binary search tree property, which states that the key in each node must be greater than all keys stored in the left sub-tree, and smaller than all keys in right sub-tree.[1] (The leaves (final nodes) of the tree contain no key and have no structure to distinguish them from one another. Leaves are commonly represented by a special leaf or nil symbol, a NULL pointer, etc.)