diff --git a/98/memo.md b/98/memo.md
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+# 98. Validate Binary Search Tree
+[リンク](https://leetcode.com/problems/validate-binary-search-tree/submissions/1952536263/)
+
+- 再帰 dfsで書いた: sol1_dfs_recursion.py
+- 色々な解法: https://github.com/mamo3gr/arai60/blob/98_validate-binary-search-tree/98_validate-binary-search-tree/memo.md
+- inorderに探索し、昇順になっているかを確認するのかでもとける
+    - inorder + 再帰
+    - https://github.com/nittoco/leetcode/pull/35/changes/BASE..cf57a354ba6d4fd06a3454283c3cec50011ce0c4#r1739978684
+    - inorder + stackで書いてみる sol3
+
+- 帰りがけ iterative
+    - https://github.com/naoto-iwase/leetcode/pull/33#discussion_r2479195403
+    - これは自分で書けそうにない
+    - 親の left or rightに新しいノードが加えられる
+
+- stack dfs
+    - https://github.com/mamo3gr/arai60/blob/98_validate-binary-search-tree/98_validate-binary-search-tree/step3.py
+    - 書いてみる： sol2
diff --git a/98/sol1_dfs_recursion.py b/98/sol1_dfs_recursion.py
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+++ b/98/sol1_dfs_recursion.py
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+from typing import Optional
+
+
+# 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 isValidBST_with_range(node, must_be_greater_than, must_be_less_than):
+            if node is None:
+                return True
+            if node.val <= must_be_greater_than or node.val >= must_be_less_than:
+                return False
+            return isValidBST_with_range(
+                node.left, must_be_greater_than, node.val
+            ) and isValidBST_with_range(node.right, node.val, must_be_less_than)
+
+        return isValidBST_with_range(root, -float("inf"), float("inf"))
diff --git a/98/sol2_dfs_stack.py b/98/sol2_dfs_stack.py
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+++ b/98/sol2_dfs_stack.py
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+from typing import Optional
+
+
+# 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:
+        if root is None:
+            return True
+
+        frontier = [(root, -float("inf"), float("inf"))]
+        while frontier:
+            node, must_be_greater_than, must_be_less_than = frontier.pop()
+            if not must_be_greater_than < node.val < must_be_less_than:
+                return False
+            if node.left is not None:
+                frontier.append((node.left, must_be_greater_than, node.val))
+            if node.right is not None:
+                frontier.append((node.right, node.val, must_be_less_than))
+
+        return True
diff --git a/98/sol3_inorder.py b/98/sol3_inorder.py
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+++ b/98/sol3_inorder.py
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+from typing import Optional
+
+
+# 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:
+        frontier = []
+
+        def push_it_and_left_children(node):
+            while node is not None:
+                frontier.append(node)
+                node = node.left
+
+        push_it_and_left_children(root)
+        min_value = -float("inf")
+        while frontier:
+            node = frontier.pop()
+            if min_value >= node.val:
+                return False
+            min_value = node.val
+            push_it_and_left_children(node.right)
+        return True