题解 | 检查是否为BST

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/

class Checker {
    bool helper(TreeNode* root, int min_val, int max_val) {
        if(!root) return true;
        if (root->val <= min_val || root->val >= max_val) return false;
        return helper(root->left, min_val, root->val) && helper(root->right, root->val, max_val);

    }

  public:
    bool checkBST(TreeNode* root) {
        // write code here
        return helper(root, INT_MIN, INT_MAX);

    }
};