leetcode 验证二叉搜索树 中等

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        auto _ = solve(root);
        return ans;
    }

    pair<int, int> solve(TreeNode *root) {      // 某一棵子树中的最小值与最大值; flag == true 表示为父亲的左二子
        if(!ans) return {-1, -1};
        if(!root) return {-1, -1};
        auto retLef = solve(root -> left);
        auto retRig = solve(root -> right);
        if(root -> left && retLef.second >= root -> val) ans = false;
        if(root -> right && retRig.first <= root -> val) ans = false;
        pair<int, int> ret;
        ret.first = root -> left ? retLef.first : root -> val;
        ret.second = root -> right ? retRig.second : root -> val;
        return ret;
    }

    bool ans = true;
};