From 8b66bda0fc9b4cd7381bdd8ada74b044153f271d Mon Sep 17 00:00:00 2001
From: tom4649 <tkaaynakiyoshi@gmail.com>
Date: Thu, 19 Mar 2026 07:47:38 +0900
Subject: [PATCH 1/4] 98. Validate Binary Search Tree

---
 98/memo.md               | 17 +++++++++++++++++
 98/sol1_dfs_recursion.py | 25 +++++++++++++++++++++++++
 98/sol2_dfs_stack.py     | 26 ++++++++++++++++++++++++++
 98/sol3_inorder.py       | 32 ++++++++++++++++++++++++++++++++
 4 files changed, 100 insertions(+)
 create mode 100644 98/memo.md
 create mode 100644 98/sol1_dfs_recursion.py
 create mode 100644 98/sol2_dfs_stack.py
 create mode 100644 98/sol3_inorder.py

diff --git a/98/memo.md b/98/memo.md
new file mode 100644
index 0000000..aa578ec
--- /dev/null
+++ b/98/memo.md
@@ -0,0 +1,17 @@
+# 98. Validate Binary Search Tree
+
+- 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
new file mode 100644
index 0000000..853ec2e
--- /dev/null
+++ b/98/sol1_dfs_recursion.py
@@ -0,0 +1,25 @@
+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, lower_bound, upper_bound):
+            if node is None:
+                return True
+            if node.val <= lower_bound or node.val >= upper_bound:
+                return False
+            return isValidBST_with_range(
+                node.left, lower_bound, min(upper_bound, node.val)
+            ) and isValidBST_with_range(
+                node.right, max(lower_bound, node.val), upper_bound
+            )
+
+        return isValidBST_with_range(root, -float("inf"), float("inf"))
diff --git a/98/sol2_dfs_stack.py b/98/sol2_dfs_stack.py
new file mode 100644
index 0000000..6e79eab
--- /dev/null
+++ b/98/sol2_dfs_stack.py
@@ -0,0 +1,26 @@
+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
+
+        frontiers = [(root, -float("inf"), float("inf"))]
+        while frontiers:
+            node, lower_bound, upper_bound = frontiers.pop()
+            if not lower_bound < node.val < upper_bound:
+                return False
+            if node.left is not None:
+                frontiers.append((node.left, lower_bound, node.val))
+            if node.right is not None:
+                frontiers.append((node.right, node.val, upper_bound))
+        return True
diff --git a/98/sol3_inorder.py b/98/sol3_inorder.py
new file mode 100644
index 0000000..54a8f9c
--- /dev/null
+++ b/98/sol3_inorder.py
@@ -0,0 +1,32 @@
+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:
+        frontiers = []
+
+        def push_it_and_left_children(node):
+            if node is None:
+                return
+            while node.left is not None:
+                frontiers.append(node)
+                node = node.left
+
+        push_it_and_left_children(root)
+        min_value = -float("inf")
+        while frontiers:
+            node = frontiers.pop()
+            if min_value >= node.val:
+                return False
+            min_value = node.val
+            if node.right is not None:
+                push_it_and_left_children(node.right)
+        return True

From 9c5a5d17a5e8a6b9c283ec6c1b7ef360294b5cc4 Mon Sep 17 00:00:00 2001
From: tom4649 <tkaaynakiyoshi@gmail.com>
Date: Thu, 19 Mar 2026 08:37:33 +0900
Subject: [PATCH 2/4] Add url

---
 98/memo.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/98/memo.md b/98/memo.md
index aa578ec..ebdb82c 100644
--- a/98/memo.md
+++ b/98/memo.md
@@ -1,4 +1,5 @@
 # 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

From f93dc40829917da3eee5c44971d591d481791a72 Mon Sep 17 00:00:00 2001
From: tom4649 <tkaaynakiyoshi@gmail.com>
Date: Thu, 26 Mar 2026 07:05:29 +0900
Subject: [PATCH 3/4] Fix sol3.py

---
 98/sol3_inorder.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/98/sol3_inorder.py b/98/sol3_inorder.py
index 54a8f9c..816a20c 100644
--- a/98/sol3_inorder.py
+++ b/98/sol3_inorder.py
@@ -14,9 +14,7 @@ def isValidBST(self, root: Optional[TreeNode]) -> bool:
         frontiers = []
 
         def push_it_and_left_children(node):
-            if node is None:
-                return
-            while node.left is not None:
+            while node is not None:
                 frontiers.append(node)
                 node = node.left
 
@@ -27,6 +25,5 @@ def push_it_and_left_children(node):
             if min_value >= node.val:
                 return False
             min_value = node.val
-            if node.right is not None:
-                push_it_and_left_children(node.right)
+            push_it_and_left_children(node.right)
         return True

From 4aa5eb7c407086d5c7610d1402465ea37a3a0640 Mon Sep 17 00:00:00 2001
From: tom4649 <tkaaynakiyoshi@gmail.com>
Date: Thu, 26 Mar 2026 07:51:05 +0900
Subject: [PATCH 4/4] Add suggested changes

---
 98/sol1_dfs_recursion.py | 10 ++++------
 98/sol2_dfs_stack.py     | 13 +++++++------
 98/sol3_inorder.py       |  8 ++++----
 3 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/98/sol1_dfs_recursion.py b/98/sol1_dfs_recursion.py
index 853ec2e..04c7bf3 100644
--- a/98/sol1_dfs_recursion.py
+++ b/98/sol1_dfs_recursion.py
@@ -11,15 +11,13 @@ def __init__(self, val=0, left=None, right=None):
 
 class Solution:
     def isValidBST(self, root: Optional[TreeNode]) -> bool:
-        def isValidBST_with_range(node, lower_bound, upper_bound):
+        def isValidBST_with_range(node, must_be_greater_than, must_be_less_than):
             if node is None:
                 return True
-            if node.val <= lower_bound or node.val >= upper_bound:
+            if node.val <= must_be_greater_than or node.val >= must_be_less_than:
                 return False
             return isValidBST_with_range(
-                node.left, lower_bound, min(upper_bound, node.val)
-            ) and isValidBST_with_range(
-                node.right, max(lower_bound, node.val), upper_bound
-            )
+                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
index 6e79eab..2ae9f3c 100644
--- a/98/sol2_dfs_stack.py
+++ b/98/sol2_dfs_stack.py
@@ -14,13 +14,14 @@ def isValidBST(self, root: Optional[TreeNode]) -> bool:
         if root is None:
             return True
 
-        frontiers = [(root, -float("inf"), float("inf"))]
-        while frontiers:
-            node, lower_bound, upper_bound = frontiers.pop()
-            if not lower_bound < node.val < upper_bound:
+        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:
-                frontiers.append((node.left, lower_bound, node.val))
+                frontier.append((node.left, must_be_greater_than, node.val))
             if node.right is not None:
-                frontiers.append((node.right, node.val, upper_bound))
+                frontier.append((node.right, node.val, must_be_less_than))
+
         return True
diff --git a/98/sol3_inorder.py b/98/sol3_inorder.py
index 816a20c..a7b924d 100644
--- a/98/sol3_inorder.py
+++ b/98/sol3_inorder.py
@@ -11,17 +11,17 @@ def __init__(self, val=0, left=None, right=None):
 
 class Solution:
     def isValidBST(self, root: Optional[TreeNode]) -> bool:
-        frontiers = []
+        frontier = []
 
         def push_it_and_left_children(node):
             while node is not None:
-                frontiers.append(node)
+                frontier.append(node)
                 node = node.left
 
         push_it_and_left_children(root)
         min_value = -float("inf")
-        while frontiers:
-            node = frontiers.pop()
+        while frontier:
+            node = frontier.pop()
             if min_value >= node.val:
                 return False
             min_value = node.val