Update README.md

Author: eMahtab

README.md

@@ -1,6 +1,36 @@
 # Validate a Binary Search Tree(BST)
+# https://leetcode.com/problems/validate-binary-search-tree
 
+Given a binary tree, determine if it is a valid binary search tree (BST).
 
+Assume a BST is defined as follows:
+
+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.
+ 
+```
+Example 1:
+
+    2
+   / \
+  1   3
+
+Input: [2,1,3]
+Output: true
+
+Example 2:
+
+    5
+   / \
+  1   4
+     / \
+    3   6
+
+Input: [5,1,4,null,null,3,6]
+Output: false
+Explanation: The root node's value is 5 but its right child's value is 4.
+```
 # Implementation :
 
 ```java