[leetcode] 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"].

https://oj.leetcode.com/problems/word-break-ii/ 

思路：DFS枚举。

dfs超时

public class Solution {
public ArrayList<String> wordBreak(String s, Set<String> dict) {
    ArrayList<String> res = new ArrayList<String>();
    if(s==null || s.length()==0)
        return res;
    helper(s,dict,0,"",res);
    return res;
}
private void helper(String s, Set<String> dict, int start, String item, ArrayList<String> res)
{
    if(start>=s.length())
    {
        res.add(item);
        return;
    }
    StringBuilder str = new StringBuilder();
    for(int i=start;i<s.length();i++)
    {
        str.append(s.charAt(i));
        if(dict.contains(str.toString()))
        {
            String newItem = item.length()>0?(item+" "+str.toString()):str.toString();
            helper(s,dict,i+1,newItem,res);
        }
    }
}
}

第二遍记录： dfs写法，依然超时。

public class Solution {
    public ArrayList<String> wordBreak(String s, Set<String> dict) {
        ArrayList<String> res = new ArrayList<String>();
        StringBuilder sb = new StringBuilder();

        wordBreakHelper(res, s, dict, sb);

        return res;
    }

    private void wordBreakHelper(ArrayList<String> res, String s, Set<String> dict, StringBuilder sb) {
        if ("".equals(s)) {
            res.add(sb.toString().substring(0, sb.length() - 1));
            return;
        }

        for (int i = 1; i <= s.length(); i++) {
            String left = s.substring(0, i);
            if (dict.contains(left)) {
                sb.append(left + " ");
                wordBreakHelper(res, s.substring(i), dict, sb);
                sb.delete(sb.length() - left.length() - 1, sb.length());
            }
        }

    }

    public static void main(String[] args) {

        String s = "catsanddog";
        String[] a = { "cat", "cats", "and", "sand", "dog" };
        Set<String> dict = new HashSet<String>();
        for (String each : a)
            dict.add(each);

        System.out.println(new Solution().wordBreak(s, dict));
    }
}