feat(work): add LeetCode solutions for 217, 271, and 98

work/1/Medium/Tree/98.validate-binary-search-tree.py

@@ -0,0 +1,62 @@
+"""
+98. Validate Binary Search Tree
+Difficulty: Medium
+https://leetcode.com/problems/validate-binary-search-tree/
+
+──────────────────────────────────────────────────
+
+Given the root of a binary tree, determine if it is a valid binary
+search tree (BST).
+
+A valid BST is defined as follows:
+
+• The left subtree of a node contains only nodes with keys strictly
+less than the node's key.
+
+• The right subtree of a node contains only nodes with keys strictly
+greater than the node's key.
+
+        • Both the left and right subtrees must also be binary search trees.
+
+
+
+Example 1:
+
+Input: root = [2,1,3]
+Output: true
+
+Example 2:
+
+Input: root = [5,1,4,null,null,3,6]
+Output: false
+Explanation: The root node's value is 5 but its right child's value
+is 4.
+
+
+
+Constraints:
+
+        • The number of nodes in the tree is in the range [1, 10^4].
+
+        • -2^31 <= Node.val <= 2^31 - 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 isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def valid(node, low, high):
+            if not node:
+                return True
+
+            if not (low < node.val < high):
+                return False
+
+            return valid(node.left, low, node.val) and valid(node.right, node.val, high)
+
+        return valid(root, float("-inf"), float("inf"))