add solution: validate-binary-search-tree

Author: dusunax

validate-binary-search-tree/dusunax.py

@@ -0,0 +1,26 @@
+'''
+# 98. Validate Binary Search Tree
+
+check if is a valid BST.
+- the left value is smaller than the current value
+- the right value is greater than the current value
+
+👉 all left and right subtrees must also valid continuously. 
+so, we need to check the low & high bound recursively. (not only check current node & immediate children)
+
+## base case
+- if the node is None, return True (empty tree is valid BST)
+- valid node value has range of left < node.val < right, if not, return False
+- recursively check the left and right subtree, update the low & high.
+'''
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def dfs(node, low = float('-inf'), high = float('inf')):
+            if not node:
+                return True
+            if not (low < node.val < high):
+                return False
+            
+            return dfs(node.left, low, node.val) and dfs(node.right, node.val, high)
+        
+        return dfs(root)