Largest BST subtree

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