diff --git a/NeetCode/valid-binary-search-tree/step1.py b/NeetCode/valid-binary-search-tree/step1.py
new file mode 100644
index 0000000..e2fdbda
--- /dev/null
+++ b/NeetCode/valid-binary-search-tree/step1.py
@@ -0,0 +1,65 @@
+"""
+Question:
+    How is the num of nodes?
+Discussion:
+    bfs
+        use deque
+    dfs
+        use stack or recursion
+
+左部分木は今までのどの親よりも小さくなければならない
+右部分木は今までのどの親よりも大きくなければならない
+
+つまり、
+    右に行く場合は、大きい分にはいいが、自分より小さいのはいけない。
+    左に行く場合は、小さい分にはいいが、自分より大きいのはいけない
+"""
+
+# 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 check(node, left, right):
+            if not node:
+                return True
+            if not (left < node.val < right):
+                return False
+            return check(node.left, left, node.val) and check(node.right, node.val, right)
+        
+        return check(root, float('-inf'), float('inf'))
+
+from collections import deque
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        node_queue = deque()
+        node_queue.append((root, float('-inf'), float('inf')))
+        while node_queue:
+            node, left, right = node_queue.pop()
+            if not node:
+                continue
+            
+            if not (left < node.val < right):
+                return False
+            node_queue.append((node.left, left, node.val))
+            node_queue.append((node.right, node.val, right))
+        return True
+
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        node_stack = []
+        node_stack.append((root, float('-inf'), float('inf')))
+        while node_stack:
+            node, left, right = node_stack.pop()
+            if not node:
+                continue
+            
+            if not (left < node.val < right):
+                return False
+            node_stack.append((node.right, node.val, right))
+            node_stack.append((node.left, left, node.val))
+        return True
diff --git a/NeetCode/valid-binary-search-tree/step3.py b/NeetCode/valid-binary-search-tree/step3.py
new file mode 100644
index 0000000..0d5cef9
--- /dev/null
+++ b/NeetCode/valid-binary-search-tree/step3.py
@@ -0,0 +1,11 @@
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def check(node, left, right):
+            if not node:
+                return True
+            
+            if not (left < node.val < right):
+                return False
+            return check(node.left, left, node.val) and check(node.right, node.val, right)
+        
+        return check(root, float('-inf'), float('inf'))