Leetcode 2135. Count Words Obtained After Adding a Letter

Count how many target words can be obtained by taking some start word, appending exactly one new letter not already present, and permuting the result — equivalently, for each target check if removing one of its letters yields a letter-set that appears among startWords. This reduces to comparing letter-sets (e.g., 26-bit masks or sets) and checking if any single-letter-deleted target-set matches a start-set.