Given a string s and a dictionary of words wordDict, determine whether s can be broken into consecutive dictionary words with no leftover characters — the same word can be reused as many times as the split calls for. The naive way tries every possible split point recursively, but the same starting position gets re-explored again and again through different earlier splits — that's what makes it exponential. Tracking, for every position, whether the prefix up to there is reachable at all (a simple boolean array) turns that repeated work into a single pass: once a position is known reachable, walking forward through a trie of the dictionary finds every word that starts there in one sweep, marking each landing spot reachable too.