Adding Efficient Solution for Problem - 98 - Validate BST

Author: Vanditg

Validate_BST/EfficientSolution.py

@@ -0,0 +1,46 @@
+##==================================
+## Leetcode
+## Student: Vandit Jyotindra Gajjar
+## Year: 2020
+## Problem: 98
+## Problem Name: 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.
+class Solution:
+    def isValid(self, root):
+        return self.BST(root, float("-inf"), float("inf"))	#Return True or False based on Helper method BST
+    def BST(self, root, minVal, maxVal):	#Defining helper method BST
+        if root is None:	#Condition-check:If root is empty
+            return True	#We return True 
+        if root.val <= minVal or root.val >= maxVal:	#Condition-check:If root's value is less than or equal to min value or greater than equal to max value
+            return False	#We return False
+        validLeft = self.BST(root.left, minVal, root.val)	#Initialize validLeft by using helper method 
+        validRight = self.BST(root.right, root.val, maxVal)	#Initialize validRight by using helper method
+        return validLeft and validRight	#If niether of is False, we return False or True for both True values