[Leo] 7th Week solutions (last question)

Author: leokim0922

validate-binary-search-tree/Leo.py

@@ -0,0 +1,12 @@
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def helper(node, low, high):
+            if not node:
+                return True
+            if not (low < node.val < high):
+                return False
+            return helper(node.left, low, node.val) and helper(node.right, node.val, high)
+
+        return helper(root, -inf, inf)
+
+        ## TC: O(n), SC: O(n)