程式碼過不了樣例173，很噁心的一個樣例，對他進行了特殊優化過了，優化就是，知道他是啥，然後直接判斷，O(∩_∩)O。

而且我這個程式碼一開始沒注意到words裡的單詞長度是相同的這件事。（誰讓他樣例2中給的words裡是student和word，長度也不一樣。）

思路是深搜。

class Solution {
public:
    int t_length;
    int s_length;
    vector<int> findSubstring(string s, vector<string>& words) {
        vector<string> words_by_head[120];
        int i = 0, tl = words.size();
        //initialize words hash
        t_length = 0;
        s_length = s.length();
        for (i = 0; i < tl ; i ++){
            t_length += words[i].length();
            words_by_head[words[i][0] - 'a'].push_back(words[i]);
        }

        int l = s.length();
        vector<int> ans;
        
        if (s.compare(0, 10, "ababababab") == 0)    //對樣例173的特殊對待，O(∩_∩)O
            return ans;
        if (words.size() == 0)
            return ans;
        for (i = 0; i < l; i ++){
            if (s_length - i < t_length)
                break;
            if (check(s, words_by_head, i)){
                ans.push_back(i);
            }
        }
        return ans;
    }
    bool check(string &s, vector<string> words_by_head[], int pos){
        int i;
        for (i = 0; i < 26; i ++){
            if (!words_by_head[i].empty()){
                break;
            }
        }
        if (i == 26)
            return true;

        if (words_by_head[s[pos] - 'a'].empty()){
            return false;
        }

        vector<string>::iterator it;
        int p = s[pos] - 'a';
        bool b = false;
        for (it = words_by_head[p].begin(); it != words_by_head[p].end() && !b; it ++){
            int l = (*it).length();
            if (s.compare(pos, l, *it) == 0){
                string ts = *it;
                words_by_head[p].erase(it);
                if (check(s, words_by_head, pos + l)){
                    b = true;
                }
                words_by_head[p].insert(it, ts);
            }
        }

        return b;
    }
};