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).

Solution: similar to obtain the statistics of numbers of words in hash table.

class Solution {
public:
    vector<int> findSubstring(string S, vector<string> &L) {
        vector<int> res;
        if (S.empty() || L.empty()) return res;
        int M = S.size(), N = L.size();
        int K = L[0].size();
        unordered_map<string, int> need;
        unordered_map<string, int> found;
        for (int i = 0; i < N; ++i)
            need[L[i]]++;
        for (int i = 0; i <= M - N * K; ++i) {
            found.clear();
            int j;
            for (j = 0; j < N; ++j) {
                string s = S.substr(i + j * K, K);
                auto it = need.find(s);
                if (it == need.end())
                    break;
                if (it->second <= found[s])
                    break;
                found[s]++;
            }
            if (j == N) res.push_back(i);
        }
        return res;
    }
};