Devise an algorithm to check if a binary tree is a binary search tree.
A simple check between left and right nodes will not suffice. Think of what happens in a deeper tree and the criteria numbers at the bottom must fulfill.
A simple check of the left and right child node will not suffice for this. Take a look at this binary tree:

Binary Tree that is not a Binary Search Tree

Because 7 is less than 9, a simple localized check between left and right nodes would not provide the correct answer. Instead we keep track of the minimum and maximum value a node can be at any point in the tree.

isBST(Node root)
    return isBSTHelper(root, INTEGER_MIN, INTEGER_MAX)

isBSTHelper(Node n, int min, int max)
    if (n == null)
       return true;

    if ( > min && < max)
       return isBSTHelper(n.left, min, && isBSTHelper(n.right,, max);

    return false;