Word Break II

Given a string s and a dictionary of words dict, add spaces in s to construct a sentence where each word is a valid dictionary word.

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"].

这道题目是word break的变种，这里要求返回可能的组合，不是单纯的判断可不可以有dict种的元素组成。我们采用回溯法，类似N-queen中的循环递归所有可能的情况。代码如下：

public class Solution {
    public List<String> wordBreak(String s, Set<String> wordDict) {
        List<String> list = new ArrayList<String>();
        if(s == null || s.length() == 0) return list;
        helper(0, "", s, wordDict, list);
        return list;
    }
    
    private void helper(int start, String res, String s, Set<String> wordDict, List<String> list) {
        if(start == s.length()) {
            list.add(res);
        }
        for(int i = start; i < s.length(); i++) {
            String tem = s.substring(start, i + 1);
            if(wordDict.contains(tem)) {
                String newS = res.length() > 0 ? res + " " + tem : tem;
                helper(i + 1, newS, s, wordDict, list);
            }
        }
    }
}