solve: validateBinarySearchTree

Author: yolophg

validate-binary-search-tree/yolophg.py

@@ -0,0 +1,21 @@
+# Time Complexity: O(n) - visit every node once during the inorder traversal
+# Space Complexity: O(n) - store all node values in an array during traversal
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        # helper to do an inorder traversal and return a list of values
+        def inorder(node):
+            if not node:
+                return []
+            # in-order: left -> current -> right
+            return inorder(node.left) + [node.val] + inorder(node.right)
+
+        # get the in-order traversal of the tree
+        arr = inorder(root)
+
+        # if there are duplicates, it's not a valid BST
+        if len(arr) != len(set(arr)):
+            return False
+        
+        # if it's sorted in strictly increasing order, it's a valid BST
+        return arr == sorted(arr)