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].
(order does not matter).

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> result;
        int wordNum = L.size();
        if (wordNum == 0) return result;
        for (int i = 0; i < wordNum; ++i)
            ++words[L[i]];
        
        int wordSize = L[0].size();
        int slength = S.size();
        for (int i = 0; i <= slength - wordSize*wordNum; ++i)
        {
            count.clear();
            int j = 0;
            for (; j < wordNum; ++j)
            {
                string w = S.substr(i+j*wordSize, wordSize); 
                if(words.find(w) == words.end())  
                    break;  
                ++count[w];  
                if(count[w] > words[w])  
                    break;  
            }
            if (j == wordNum) result.push_back(i);
        }
        return result;
    }
};