added validate bst

14. Questions/leetcode 98 - validate binary search tree.py

@@ -0,0 +1,36 @@
+# validate binary search tree | leetcode 98 | https://leetcode.com/problems/validate-binary-search-tree/
+# Given the root of a binary tree, determine if it is a valid binary search tree (BST).
+# method: in-order traversal of a valid bst gives a sorted array
+# tip: use `prev` pointer instead of an array to keep space complexity as O(1)
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+
+    # initialise a prev pointer
+    def __init__(self):
+        self.prev = None
+
+    # in-order traversal (L M R)
+    # should return a sorted array
+    def isValidBST(self, root) -> bool:
+
+        # if this node is none, its a leaf
+        if root is None:
+            return True
+
+        if not self.isValidBST(root.left):
+            return False
+
+        if self.prev is not None and self.prev.val >= root.val:
+            return False
+
+        self.prev = root
+
+        return self.isValidBST(root.right)