LC 98. Validate Binary Search Tree

https://leetcode.com/problems/validate-binary-search-tree/description/?envType=study-plan-v2&envId=top-interview-150

typedef long long ll;
class Solution {
public:
    bool dfs(TreeNode* cur, ll l, ll r) {
        if (!cur) return true;
        if (cur -> val <= l || r <= cur -> val) return false;
        return dfs(cur -> left, l, cur -> val) && dfs(cur -> right, cur -> val, r);
    }
    bool isValidBST(TreeNode* root) {
        return dfs(root, LLONG_MIN, LLONG_MAX);
    }
};

Time Complexity: O(n)
Space: O(1)

也是一題很好的 DFS 基礎題目