It's a constant upper bound for lg N, which is shown in the editorial for the maximum subword length. It's easy to remember 2^20 > 1e6 > 1e5, which helps simpli

It's a constant upper bound for lg N, which is shown in the editorial for the maximum subword length. It's easy to remember 2^20 > 1e6 > 1e5, which helps simplify the thought process behind the implementation.