30 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) {
        vector<int> res;
        if (L.size()<=0)
            return res;
        int wordCount = L.size();
        int wordSize = L[0].size();
        if (S.size() < wordCount*wordSize)
            return res;
        unordered_map<string, int> mp, mp_copy;
        for (int i=0;i<L.size();i++)
        {
            mp_copy[L[i]]++;
        }
        for (int start=0;start<=S.size()-wordCount*wordSize; ++start)
        {
            mp=mp_copy;
            bool exist = true;
            for (int j=0;j<wordCount;j++)
            {
                string cur = S.substr(start+j*wordSize, wordSize);
                if (mp.find(cur)==mp.end() || mp[cur]<=0)
                {
                    exist = false;
                    break;
                }
                else
                {
                    mp[cur]--;
                }
            }
            if (exist)
            {
                res.push_back(start);
            }
        }
        return res;
    }
    
};