diff --git a/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_1.py b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_1.py
new file mode 100644
index 0000000..e313f4b
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_1.py	
@@ -0,0 +1,14 @@
+# DFS(再帰)
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+
+        def helper_valid_bst(node, min_val=-math.inf, max_val=math.inf):
+            if not node:
+                return True
+            if not min_val < node.val < max_val:
+                return False
+            return helper_valid_bst(
+                node.left, min_val, node.val
+            ) and helper_valid_bst(node.right, node.val, max_val)
+
+        return helper_valid_bst(root)
diff --git a/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_2.py b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_2.py
new file mode 100644
index 0000000..a65e401
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_2.py	
@@ -0,0 +1,46 @@
+# DFS(再帰)
+# 再帰部分の戻り値を変数に格納
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def helper_valid_bst(node, lower=-math.inf, upper=math.inf):
+            if not node:
+                return True
+            if not lower < node.val < upper:
+                return False
+            if not helper_valid_bst(node.left, lower, node.val):
+                return False
+            if not helper_valid_bst(node.right, node.val, upper):
+                return False
+            return True
+
+        return helper_valid_bst(root)
+
+
+# DFS(スタック)
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        node_range_stack = [(root, -math.inf, math.inf)]
+        while node_range_stack:
+            node, lower, upper = node_range_stack.pop()
+            if not node:
+                continue
+            if not lower < node.val < upper:
+                return False
+            node_range_stack.append((node.right, node.val, upper))
+            node_range_stack.append((node.left, lower, node.val))
+        return True
+
+
+# BFS
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        node_range_queue = deque([(root, -math.inf, math.inf)])
+        while node_range_queue:
+            node, lower, upper = node_range_queue.popleft()
+            if not node:
+                continue
+            if not lower < node.val < upper:
+                return False
+            node_range_queue.append((node.left, lower, node.val))
+            node_range_queue.append((node.right, node.val, upper))
+        return True
diff --git a/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_3.py b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_3.py
new file mode 100644
index 0000000..fad965f
--- /dev/null
+++ b/arai60/21-29_Tree_BT_BST/28_98_Validate Binary Search Tree/level_3.py	
@@ -0,0 +1,14 @@
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def helper_valid_bst(node, lower=-math.inf, upper=math.inf):
+            if not node:
+                return True
+            if not lower < node.val < upper:
+                return False
+            if not helper_valid_bst(node.left, lower, node.val):
+                return False
+            if not helper_valid_bst(node.right, node.val, upper):
+                return False
+            return True
+
+        return helper_valid_bst(root)