adding isValidBST

Author: daiyongg-kim

validate-binary-search-tree/daiyongg-kim.py

@@ -0,0 +1,18 @@
+# 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:
+        
+        def checkBST(root, min_val, max_val):
+            if not root:
+                return True
+            
+            if not (min_val < root.val < max_val):
+                return False
+            return checkBST(root.left, min_val, root.val) and checkBST(root.right, root.val, max_val)
+        
+        return checkBST(root, float('-inf'), float('inf'))