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> ret;
        int lsize=L.size();
        if(lsize==0) return ret;

        int slen=S.length();
        int wordlen=L[0].length();
        
        map<string,int> lmap;
        for(int i=0;i<lsize;i++)
            if(lmap.find(L[i])==lmap.end())
                lmap[L[i]]=1;
            else
                lmap[L[i]]++;

        for(int i=0;i<slen-lsize*wordlen+1;i++)
        {        
            map<string,int> imap;
            bool valid=true;
            for(int j=0;j<lsize;j++)
            {
                int start=i+j*wordlen;
                string word=S.substr(start,wordlen);
                imap[word]++;
                if(lmap.find(word)==lmap.end() || lmap[word]<imap[word])
                {
                    valid=false;
                    break;
                }
                
            }
            if(!valid) continue;
            else
                ret.push_back(i);
        }
        return ret;
    }
};