From 5737219a73570107b3bfe0f7557e77f3e7b60d90 Mon Sep 17 00:00:00 2001
From: Yu Nakamura <nkmr357@gmail.com>
Date: Tue, 21 Apr 2026 10:18:04 +0900
Subject: [PATCH] add files

---
 0098-validate-binary-search-tree/memo.md   | 11 +++++++++
 0098-validate-binary-search-tree/step1.cpp | 28 ++++++++++++++++++++++
 0098-validate-binary-search-tree/step2.cpp | 28 ++++++++++++++++++++++
 0098-validate-binary-search-tree/step3.cpp | 28 ++++++++++++++++++++++
 4 files changed, 95 insertions(+)
 create mode 100644 0098-validate-binary-search-tree/memo.md
 create mode 100644 0098-validate-binary-search-tree/step1.cpp
 create mode 100644 0098-validate-binary-search-tree/step2.cpp
 create mode 100644 0098-validate-binary-search-tree/step3.cpp

diff --git a/0098-validate-binary-search-tree/memo.md b/0098-validate-binary-search-tree/memo.md
new file mode 100644
index 0000000..0af37da
--- /dev/null
+++ b/0098-validate-binary-search-tree/memo.md
@@ -0,0 +1,11 @@
+### step1
+
+dfsでmin, maxを更新しながら左の葉の方がmaxと同じか大きいか、右の葉の方がminと同じか小さいかでfalseを返すように実装。
+
+### step2
+
+変更点なし。
+
+### step3
+
+3回通すまで書き直し。
diff --git a/0098-validate-binary-search-tree/step1.cpp b/0098-validate-binary-search-tree/step1.cpp
new file mode 100644
index 0000000..11c30bc
--- /dev/null
+++ b/0098-validate-binary-search-tree/step1.cpp
@@ -0,0 +1,28 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool traverse(TreeNode* root, long long min, long long max) {
+        if (!root) {
+            return true;
+        }
+        if (root->val >= max || root->val <= min) {
+            return false;
+        }
+        return traverse(root->left, min, root->val) 
+            && traverse(root->right, root->val, max);
+    }
+
+    bool isValidBST(TreeNode* root) {
+        return traverse(root, LLONG_MIN, LLONG_MAX);
+    }
+};
diff --git a/0098-validate-binary-search-tree/step2.cpp b/0098-validate-binary-search-tree/step2.cpp
new file mode 100644
index 0000000..11c30bc
--- /dev/null
+++ b/0098-validate-binary-search-tree/step2.cpp
@@ -0,0 +1,28 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool traverse(TreeNode* root, long long min, long long max) {
+        if (!root) {
+            return true;
+        }
+        if (root->val >= max || root->val <= min) {
+            return false;
+        }
+        return traverse(root->left, min, root->val) 
+            && traverse(root->right, root->val, max);
+    }
+
+    bool isValidBST(TreeNode* root) {
+        return traverse(root, LLONG_MIN, LLONG_MAX);
+    }
+};
diff --git a/0098-validate-binary-search-tree/step3.cpp b/0098-validate-binary-search-tree/step3.cpp
new file mode 100644
index 0000000..11c30bc
--- /dev/null
+++ b/0098-validate-binary-search-tree/step3.cpp
@@ -0,0 +1,28 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool traverse(TreeNode* root, long long min, long long max) {
+        if (!root) {
+            return true;
+        }
+        if (root->val >= max || root->val <= min) {
+            return false;
+        }
+        return traverse(root->left, min, root->val) 
+            && traverse(root->right, root->val, max);
+    }
+
+    bool isValidBST(TreeNode* root) {
+        return traverse(root, LLONG_MIN, LLONG_MAX);
+    }
+};