Leetcode 17.15 最长单词 剪枝与记忆化

　　主函数的剪枝，判断单词是否由数组中的单词组成的函数的记忆化。

public final String longestWord(String[] words) {
        Arrays.sort(words, (o1, o2) -> {
            if (o1.length() == o2.length()) return o1.compareTo(o2);
            else return Integer.compare(o2.length(), o1.length());
        });
        Set<String> set = new HashSet<String>();
        for (String word : words) set.add(word);
        for (String word : words) {
            set.remove(word);
            if (isCompose(word, word.length(), set, new int[word.length() + 1])) return word;
        }
        return "";
    }

    private final boolean isCompose(String target, int point, Set<String> set, int[] cache) {
        if (point == 0) return true;
        if (cache[point] != 0) return cache[point] == 1;
        for (int i = point - 1; i >= 0; i--) {
            String last = target.substring(i, point);
            if (set.contains(last) && isCompose(target, i, set, cache)) {
                cache[point] = 1;
                break;
            } else cache[point] = -1;
        }
        return cache[point] == 1;
    }