#98: Validate Binary Search Tree

Author: papilo-cloud

Trees/is_valid_bst.py

@@ -0,0 +1,44 @@
+# 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.
+ 
+
+# Constraints:
+
+# The number of nodes in the tree is in the range [1, 104].
+# -231 <= Node.val <= 231 - 1
+
+
+from typing import Optional
+# 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:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        return self.checkValidity(root, float('-inf'), float('inf'))
+
+    def checkValidity(self, node, min_val, max_val):
+        if not node:
+            return True
+        if node.val < min_val or node.val > max_val:
+            return False
+
+        return self.checkValidity(node.left, min_val, node.val - 1) and \
+                self.checkValidity(node.right, node.val + 1, max_val)