Word Break

Given a string s and a dictionary of words dict, determine if s can be segmented into a space-separated sequence of one or more dictionary words.

For example, given
s = "leetcode",
dict = ["leet", "code"].

Return true because "leetcode" can be segmented as "leet code"

给一个字符串s和一个字典dict，编程实现s能否由dict中组合而成。

方案一的DFS提交TLE，方案二使用动态规划

class Solution{
private:
    void helper(string s,unordered_set<string>& wordDict,int start,bool& res){
        if(start == int(s.length())){
            res = true;
            return;
        }
        for(int i = start;i<int(s.size());i++){
            string word = s.substr(start,i - start + 1);
            if(wordDict.find(word) != wordDict.end() && !res){

                helper(s,wordDict,i+1,res);
            }
        }
    }

public:
    bool wordBreak(string s,unordered_set<string>& wordDict){
        bool res = false;
        helper(s,wordDict,0,res);
        return res;
    }
};

class Solution2{
public:
    bool wordBreak(string s,unordered_set<string>& wordDict){
        vector<bool> res(s.size()+1,false);
        res[0] = true;

        for(int i=0;i<int(s.size())+1;i++){
            for(int j=0;j<i;j++){
                if(res[j] && wordDict.find(s.substr(j,i-j)) != wordDict.end()){
                    res[i] = true;
                    break;
                }
            }
        }
        return res.back();
    }
};