added the new question for validating binary search tree

Twopointer/Binary search/Linked List/Trees/98. Validate Binary Search Tree.py

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+'''98. Validate Binary Search Tree
+Medium
+
+Given the root of a binary tree, determine if it is a valid binary search tree (BST).
+
+A valid 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:
+
+
+Input: root = [2,1,3]
+Output: true
+Example 2:
+
+
+Input: root = [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.'''
+
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def isValidBST(self, root):
+        def helper(node, lower=float('-inf'), upper=float('inf')):
+            if not node:
+                return True
+
+            val = node.val
+            if val <= lower or val >= upper:
+                return False
+
+            if not helper(node.right, val, upper):
+                return False
+            if not helper(node.left, lower, val):
+                return False
+            return True
+
+        return helper(root)