leetcode_Substring with Concatenation of All Words

You are given a string, S, and a list of words, L, that are all of the same length. Find all starting indices of substring(s) in S that is a concatenation of each word in L exactly once and without any intervening characters.

For example, given:
S: "barfoothefoobarman"
L: ["foo", "bar"]

You should return the indices: [0,9].

　　

class Solution {
public:
    vector<int> findSubstring(string S, vector<string> &L) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        map<string, int> words;
        map<string, int> count;
        vector<int> res;
        if(L.size() == 0 || S.length() < L[0].size()*L.size() ) return res;
        for(int i = 0; i< L.size(); ++i){
            ++words[L[i]];
        }
        
        for(int i= 0; i <= S.length()- L[0].size()*L.size(); ++i)
        {
            count.clear();
            int j;
            for(j = 0; j <L.size();++j){
                string str = S.substr(i+j*L[0].size(), L[0].size());
                if(words.find(str) == words.end()) break;
                count[str]++;
                if(count[str] > words[str]) break;
            }
            if(j == L.size())
              res.push_back(i);
        }
        return res;
    }
};