validate binary search tree solution

Author: yuseok89

validate-binary-search-tree/yuseok89.py

@@ -0,0 +1,23 @@
+# TC: O(N)
+# SC: 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:
+
+    def isValid(self, left_bound, right_bound, root):
+        if root is None:
+            return True
+        if left_bound is not None and left_bound >= root.val:
+            return False
+        if right_bound is not None and right_bound <= root.val:
+            return False
+
+        return self.isValid(left_bound, root.val, root.left) and self.isValid(root.val, right_bound, root.right)
+
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        return self.isValid(None, root.val, root.left) and self.isValid(root.val, None, root.right)
+