140. 单词拆分 II

输入:
s = "catsanddog"
wordDict = ["cat", "cats", "and", "sand", "dog"]
输出:
[
  "cats and dog",
  "cat sand dog"
]

输入:
s = "pineapplepenapple"
wordDict = ["apple", "pen", "applepen", "pine", "pineapple"]
输出:
[
  "pine apple pen apple",
  "pineapple pen apple",
  "pine applepen apple"
]
解释: 注意你可以重复使用字典中的单词。

输入:
s = "catsandog"
wordDict = ["cats", "dog", "sand", "and", "cat"]
输出:
[]

class Solution {
    public List<String> wordBreak(String s, List<String> wordDict) {
        List<String> res = new ArrayList<>();
        List<String> list = new ArrayList<>();
        dfs(res, list, s, wordDict, 0);
        return res;
    }

    private void dfs(List<String> res, List<String> list, String s, List<String> wordDict, int start) {
        if (start == s.length()) {
            StringBuilder sb = new StringBuilder();
            for (String ss : list) {
                sb.append(ss);
                sb.append(" ");
            }
            res.add(sb.toString().substring(0, sb.length() - 1));
            return;
        }
        for (int i = start; i < s.length(); i++) {
            String sub = s.substring(start, i + 1);
            if (wordDict.contains(sub)) {
                list.add(sub);
                dfs(res, list, s, wordDict, i + 1);
                list.remove(list.size() - 1);
            }
        }
    }
}