98 验证二叉搜索树

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xn08xg/

https://leetcode.cn/problems/validate-binary-search-tree/description/

cost: 20m15s

/**
 * 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) {
        auto [_, _2, valid] = this->isValidBST_(root);
        return valid;
    }

private:
    tuple<int, int, bool> isValidBST_(TreeNode* root) {
        if (root == nullptr) {
            return {0, 0, true};
        }
        if (root->left == nullptr && root->right == nullptr) {
            return {root->val, root->val, true};
        }
        int minVal = root->val;
        int maxVal = root->val;
        if (root->left != nullptr) {
            auto [min, max, valid] = this->isValidBST_(root->left);
            if (!valid || max >= root->val) {
                return {0, 0, false};
            }
            minVal = min;
        }
        if (root->right != nullptr) {
            auto [min, max, valid] = this->isValidBST_(root->right);
            if (!valid || min <= root->val) {
                return {0, 0, false};
            }
            maxVal = max;
        }
        return {minVal, maxVal, true};
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(1)