A binary search tree (BST) is a node based binary tree data structure which has the following properties.
The left subtree of a node contains only nodes with keys less than the node’s key.
The right subtree of a node contains only nodes with keys greater than the node’s key.
Both the left and right subtrees must also be binary search trees.
Related Article : https://www.geeksforgeeks.org/a-program-to-check-if-a-binary-tree-is-bst-or-not/