--- tags: leetcode --- # [98. Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/) --- # My Solution ## The Key Idea for Solving This Coding Question ## C++ Code ```cpp= /** * 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 isValidBST(TreeNode *root) { return isValidBST(root, LONG_MIN, LONG_MAX); } private: bool isValidBST(TreeNode *root, long lowerBound, long upperBound) { if (root == nullptr) { return true; } if (root->val <= lowerBound || upperBound <= root->val) { return false; } return isValidBST(root->left, lowerBound, root->val) && isValidBST(root->right, root->val , upperBound); } }; ``` ## Time Complexity $O(n)$ $n$ is the number of nodes in the binary tree referred by `root`. ## Space Complexity $O(H)$ $H$ is the height of the binary tree referred by `root`. # Miscellane <!-- # Test Cases ``` [2,1,3] ``` ``` [5,1,4,null,null,3,6] ``` * Wrong Answer: ``` [2,2,2] ``` * Wrong Answer: ``` [32,26,47,19,null,null,56,null,27] ``` * Wrong Answer: ``` [1,null,1] ``` ``` [1] ``` ``` [10,5,15,null,null,6,20] ``` ``` [1,2,3,4,null,null,5] ``` ``` [3,9,20,null,null,15,7] ``` ``` [4,2,7,1,3] ``` ``` [18,2,22,null,null,null,63,null,84,null,null] ``` ``` [2147483647] ``` -->