[LeetCode] 140. Word Break II java

题目：

Given a non-empty string s and a dictionary wordDict containing a list of non-empty words, add spaces in s to construct a sentence where each word is a valid dictionary word. You may assume the dictionary does not contain duplicate words.

Return all such possible sentences.

For example, given
s = "catsanddog",
dict = ["cat", "cats", "and", "sand", "dog"].

A solution is ["cats and dog", "cat sand dog"].

题意及分析：给出一个字符串和一个字典，求能用字典里的单词将字符串分割的所有可能。使用深度遍历的方法，每次判断字符串是否以字典中的单词为开头，如果是开头，继续判断剩余的字符串；如果最后字符串长度为0，那么就找到了能分割字符串的单词组成。这里用一个hashMap保存中间结果，否则会超时。

代码：

class Solution { 
    public List<String> wordBreak(String s, List<String> wordDict) {
        return DFS(s, wordDict, new HashMap<String, LinkedList<String>>());
    }

    List<String> DFS(String s,List<String> wordDict,HashMap<String,LinkedList<String>> map){
        if(map.containsKey(s))
            return map.get(s);
        LinkedList<String> res = new LinkedList<>();
        if(s.length() == 0){
            res.add("");
            return res;
        }
        for(String word : wordDict){
            if(s.startsWith(word)){
                List<String> subList = DFS(s.substring(word.length()),wordDict,map);
                for(String sub : subList){
                    res.add(word + (sub.isEmpty() ? "":" ")+ sub);
                }
            }
        }
        map.put(s,res);
        return res;
    }
}