Create validBinarySearchTree.cpp

DFS/validBinarySearchTree.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 dfs(TreeNode *root, TreeNode *low, TreeNode *high)
+    {
+        if (!root)
+            return true;
+
+        if (low and root->val and root->val <= low->val)
+            return false;
+
+        if (high and root->val and root->val >= high->val)
+            return false;
+
+        return dfs(root->left, low, root) and dfs(root->right, root, high);
+    }
+
+    bool isValidBST(TreeNode *root)
+    {
+        return dfs(root, nullptr, nullptr);
+    }
+};