题解 | #判断是不是二叉搜索树#

bool isValidBSTHelper(struct TreeNode* node, long long lower, long long upper) {
    if (node == NULL) return true;
    if (node->val <= lower || node->val >= upper) return false;
    return isValidBSTHelper(node->left, lower, node->val) && isValidBSTHelper(node->right, node->val, upper);
}
bool isValidBST(struct TreeNode* root ) {
    return isValidBSTHelper(root, -10000000000, 10000000000);
}