validate binary search tree

Author: devyejin

validate-binary-search-tree/devyejin.py

@@ -0,0 +1,51 @@
+# Definition for a binary tree node.
+from collections import deque
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+def list_to_tree(arr):
+    if not arr:
+        return None
+#      -> 레벨로 주고 있음
+    root = TreeNode(arr[0])
+    queue = deque([root]) # 큐에 root를 넣고 시작 queue = deque() queue.append(root)와 동일
+    i = 1
+
+    while queue and i < len(arr):
+        current = queue.popleft()
+
+        if i < len(arr) and arr[i] is not None:
+            current.left = TreeNode(arr[i])
+            queue.append(current.left)
+        i += 1
+
+        if i < len(arr) and arr[i] is not None:
+            current.right = TreeNode(arr[i])
+            queue.append(current.right)
+        i += 1
+
+    return root
+
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        self.prev = None
+        def inorder(node):
+            if not node:
+                return True
+
+            # 왼쪽 서브트리 확인
+            if not inorder(node.left):
+                return False
+
+            # 현재 노드 값 확인
+            if self.prev is not None and node.val <= self.prev:
+                return False
+            self.prev = node.val
+
+            return inorder(node.right)
+
+        return inorder(root)