LeetCode Substring with Concatenation of All Words

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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) {
         map<string, int> words;
         map<string, int> curStr;
         for(int i = 0; i < L.size(); ++i)
             ++words[L.at(i)];
         int N = L.size();
         vector<int> ret;
         if(N <= 0)   return ret;
         int M = L.at(0).size();
         for(int i = 0; i <= (int)S.size()-N*M; ++i)
         {
             curStr.clear();
             int j = 0;
             for(j = 0; j < N; ++j)
             {
                 string w = S.substr(i+j*M, M);
                 if(words.find(w) == words.end())
                     break;
                 ++curStr[w];
                 if(curStr[w] > words[w])
                     break;
             }
             if(j == N)  ret.push_back(i);
         }
         return ret;
     }
 };