Add solution for 19. Remove Nth Node From End of List

Author: sbk1014

validate-binary-search-tree/bhyun-kim.py

@@ -0,0 +1,77 @@
+"""
+98. Validate Binary Search Tree
+https://leetcode.com/problems/validate-binary-search-tree/
+
+
+"""
+from typing import Optional
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+    
+"""
+Solution1:
+    Recursion
+    In the given problem, the subtree of a node has a range of values according to the previous nodes. 
+    Thus, we can define a function that checks the validity of the subtree of a node with the range of values.
+
+    - Define a function that checks the validity of the subtree of a node with the range of values
+    - Check the validity of the left subtree and the right subtree
+        - with the range of values that the left subtree and the right subtree should have
+        - If left < root < right, the subtree is valid
+    - If the left subtree and the right subtree are valid, call the function recursively for the left and right subtrees.
+        - before calling the function, update the range of values for the left and right subtrees
+    - If the left subtree and the right subtree are valid, return True
+
+Time complexity: O(N)
+    - The function is called recursively for each node
+
+Space complexity: O(N)
+    - The function stores the range of values for each node
+"""
+
+class Solution1:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        maximum = float('inf')
+        minimum = float('-inf')
+        return self.isValidSubTree(
+            root, maximum, minimum
+        )
+
+    def isValidSubTree(self, root, maximum, minimum):
+        if root.left is not None: 
+            if root.val <= root.left.val: 
+                return False
+            if not minimum < root.left.val < maximum:
+                return False 
+
+        if root.right is not None :
+            if root.val >= root.right.val: 
+                return False
+            if not minimum < root.right.val < maximum:
+                return False 
+
+        if root.left is not None:
+            l_max = min(maximum, root.val)
+            is_left_valid = self.isValidSubTree(
+                root.left, l_max, minimum
+                )
+        else:
+            is_left_valid = True
+
+        if root.right is not None:
+            r_min = max(minimum, root.val)
+            is_right_valid = self.isValidSubTree(
+                root.right, maximum, r_min 
+                )
+        else:
+            is_right_valid = True
+        
+        if is_left_valid and is_right_valid:
+            return True
+        else: 
+            return False