98. Validate Binary Search Tree

0098-validate-binary-search-tree/memo.md

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+### 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。

0098-validate-binary-search-tree/step1.cpp

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+/**
+ * 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);
+    }
+};

0098-validate-binary-search-tree/step2.cpp

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+/**
+ * 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);
+    }
+};

0098-validate-binary-search-tree/step3.cpp

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+/**
+ * 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);
+    }
+};