b.upperbound takes $$$O(logn)$$$ while upperbound(all(b), a[i]) takes $$$O(n)$$$ time.

for sets , b.upper_bound is O(log(n)) but upper_bound(all(b)) is O(n)

Using set.upper_bound() is O(log n), while std::upper_bound(set.begin(), set.end(), value) adds O(n) complexity due to linear search, potentially causing time limit.