Check for BST (GFG)

pair<int, int> compute(Node* root, bool &ans) {
    if(root == nullptr) {
        return {INT_MAX, INT_MIN};
    }
    pair<int, int> a = compute(root -> left, ans);
    pair<int, int> b = compute(root -> right, ans);
    if(root -> data <= a.second || root -> data >= b.first) {
        ans = false;
    }
    return {min(a.first, root -> data), max(b.second, root -> data)};
}

//Function to check whether a Binary Tree is BST or not.
bool isBST(Node* root) 
{
    // Your code here
    bool ans = true;
    compute(root, ans);
    return ans;
}