validate-binary-search-tree solutions

Author: hyeri0903

validate-binary-search-tree/hyeri0903.py

@@ -0,0 +1,35 @@
+# 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:
+        """
+        checking validation using inorder traversal
+        always greater than previous node's key
+
+        time complexity: O(n)
+
+        """
+
+        prev = [None]
+
+        def inorder(node):
+            if not node:
+                return True
+
+            #search left sub tree first
+            if not inorder(node.left):
+                return False
+            
+            if prev[0] is not None and node.val <= prev[0]:
+                return False
+
+            #set current node value
+            prev[0] = node.val
+            #search right sub tree
+            return inorder(node.right)
+
+        return inorder(root)