Word Break

Question

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".

My Solusion

class Solution {
public:
    bool wordBreak(string s, unordered_set<string>& wordDict) {
        /* 動態規劃解
        初始化：
        F(0, i): 表示substr[0,i)是否可分割
            則：F(0, N) = ||_{i=1}^{i=N}{F(0, i) && F(i, N)}
        邊界條件：s爲wordDict一個值
        */
        if(wordDict.find(s) != wordDict.end())
            return true;
        
        int len = s.size();
        vector<int> f(len, -1);
        
        for(int i = 1; i < len; ++i){
            string tmp = s.substr(0, i);
            if(wordDict.find(tmp) != wordDict.end() && wordBreak(s, i, wordDict, f))    // F(0, i) && F(i, N)
                return true;
        }
        return false;
        
    }
    
    bool wordBreak(string s, int beg, unordered_set<string>& wordDict, vector<int>& f)
    {
        
        
        if(f[beg] != -1) 
        {
            return f[beg];       //加速
        }
        
        bool rst = false;
        if(wordDict.find(s.substr(beg)) != wordDict.end()) // 邊界條件
        {
            rst = true;
        }else
        {
            for(int i = 1; i < s.size() - beg; ++i){
                if(wordDict.find(s.substr(beg, i)) != wordDict.end() && wordBreak(s, beg + i, wordDict, f))    // F(beg, i) && F(i, N)
                {
                    rst = true;
                    break;
                }
            }
        }
        f[beg] = rst;
        return rst;
    }
};