AtCoder ABC455-E Review: Counting Prefix Differences and Three-Set Inclusion–Exclusion
A review of a substring-counting problem, moving from brute-force enumeration to counting prefix-difference states and using inclusion–exclusion to handle repe…
AtCoder ABC457-E Review: Interval Unions, Structural Cases, and an Offline Fenwick Tree
A review of exact coverage by interval unions. Model two pieces of cloth covering a query interval exactly as an interval-union test, split valid arrangements…
AtCoder ABC458-E Review: Separators, Nonempty Groups, and Vandermonde's Convolution
A review of a combinatorial counting problem, starting from an adjacency constraint, turning an awkward arrangement condition into a structure we can enumerate…