http://www.geeksforgeeks.org/find-the-largest-subtree-in-a-tree-that-is-also-a-bst/
Given a Binary Tree, write a function that returns the size of the largest subtree which is also a Binary Search Tree (BST). If the complete Binary Tree is BST, then return the size of whole tree.
Examples:
Input:
5
/ \
2 4
/ \
1 3
Output: 3
The following subtree is the maximum size BST subtree
2
/ \
1 3
Input:
50
/ \
30 60
/ \ / \
5 20 45 70
/ \
65 80
Output: 5
The following subtree is the maximum size BST subtree
60
/ \
45 70
/ \
65 80
Solution. If a node is a subtree, max value from left subtree should be less than node.val and min value from right subtree should be greater than node.val.
Define a Triplet for (max, min, ans). If ans is negative, it means subtree is not a BST. The largest BST in subtree is abs(ans).
Check my code on github: link