A program to check if a binary tree is BST or not

Problem:
 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.
From the above properties it naturally follows that:
• Each node (item in the tree) has a distinct key.

Solution:

• Start from root with min = -INF and max = + INF
• At each node check whether node's data is in range [min,max]
• If not it violates binary search tree,return 0. 
• you can use the below isbst() trace,

 
Complexity: worst case - when it is a bst, O(n) where n is the  number of nodes.
Code:

int isBSTUtil(Tnode *root,int min,int max)
{
    if(root==NULL)
        return 1;
    if(root->data < min || root->data >= max)
        return 0;
    return isBSTUtil(root->left,min,root->data) && isBSTUtil(root->right, root->data + 1, max);
}
 
int isBST(Tnode *root)
{
    int min=1<<((sizeof(int)*8)-1);
    int max=~min;
    //printf("min=%d max=%d",min,max);
    
    return isBSTUtil(root,min,max);
}