18.7 一组字符串里找最长的可以用其他字符串表示的字符串。

用HashMap缓存记录结果，以防止重复计算。

枚举分割点，dfs。

注意单词本身不能由单词本身构成，需要isOrigin符号判断下。

import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;

public class Solution {

    public static String getLongestWord(String[] list) {
        HashMap<String, Boolean> map = new HashMap<String, Boolean>();
        for (String str : list) {
            map.put(str, true);
        }

        Arrays.sort(list, new Comparator<String>() {

            public int compare(String a, String b) {
                return ((Integer) b.length()).compareTo(a.length());
            }

        });

        for (String each : list) {
            if (canBuild(map, each, list, true))
                return each;
        }

        return "";
    }

    private static boolean canBuild(HashMap<String, Boolean> map, String word, String[] list, boolean isOriginWord) {
        if (map.containsKey(word) && !isOriginWord)
            return map.get(word);
        for (int i = 1; i < word.length(); i++) {
            String left = word.substring(0, i);
            if (map.containsKey(left) && map.get(left) == true) {
                if (canBuild(map, word.substring(i), list, false))
                    return true;
            }
        }
        map.put(word, false);
        return false;

    }

    public static void main(String[] args) {
        String[] list = { "abc", "bc", "c", "b" };
        System.out.println(getLongestWord(list));
    }
}