0098. Validate Binary Search Tree.md

/**
 * 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) {
        TreeNode* prev = nullptr;

        function<bool(TreeNode*)> dfs = [&](TreeNode* node) -> bool {
            if(!node) return true;
            if(!dfs(node->left)) return false;

            if(prev && node->val <= prev->val) return false;
            prev = node;
            return dfs(node->right);
        };

        return dfs(root);
    }
};

T.C: O(n) S.C: O(h)