Added program to check if a tree is valid BST or not.

data_structures/Trees/Valid BST or not.java

@@ -0,0 +1,62 @@
+class Node
+{
+    int data;
+    Node left, right;
+
+    public Node(int item)
+    {
+        data = item;
+        left = right = null;
+    }
+}
+
+public class BinaryTree
+{
+    //Root of the Binary Tree
+    Node root;
+
+    /* can give min and max value according to your code or
+    can write a function to find min and max value of tree. */
+
+    /* returns true if given search tree is binary
+     search tree (efficient version) */
+    boolean isBST()  {
+        return isBSTUtil(root, Integer.MIN_VALUE,
+                               Integer.MAX_VALUE);
+    }
+
+    /* Returns true if the given tree is a BST and its
+      values are >= min and <= max. */
+    boolean isBSTUtil(Node node, int min, int max)
+    {
+        /* an empty tree is BST */
+        if (node == null)
+            return true;
+
+        /* false if this node violates the min/max constraints */
+        if (node.data < min || node.data > max)
+            return false;
+
+        /* otherwise check the subtrees recursively
+        tightening the min/max constraints */
+        // Allow only distinct values
+        return (isBSTUtil(node.left, min, node.data-1) &&
+                isBSTUtil(node.right, node.data+1, max));
+    }
+
+    /* Driver program to test above functions */
+    public static void main(String args[])
+    {
+        BinaryTree tree = new BinaryTree();
+        tree.root = new Node(4);
+        tree.root.left = new Node(2);
+        tree.root.right = new Node(5);
+        tree.root.left.left = new Node(1);
+        tree.root.left.right = new Node(3);
+
+        if (tree.isBST())
+            System.out.println("IS BST");
+        else
+            System.out.println("Not a BST");
+    }
+}