Substring with Concatenation of All Words

You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) in s that is a concatenation of each word in words exactly once and without any intervening characters.

For example, given:
s: "barfoothefoobarman"
words: ["foo", "bar"]

You should return the indices: [0,9].
(order does not matter).

class Solution {
public:
    vector<int> findSubstring(string s, vector<string> & words) {
        int nums = words.size(), n = s.length(), len = words[0].length();
        vector<int> ret;

        unordered_map<string, int> count;
        for (string word : words)
            count[word]++;

        for (int i = 0; i < n - len * nums + 1; i++) {
            unordered_map<string, int> seen;
            int j = 0;
            for (; j < nums; j++) {
                string str = s.substr(i + j * len, len);
                if (count.find(str) != count.end()) {
                    seen[str]++;
                    if (seen[str] > count[str])
                        break;
                }else{
                    break;
                }
            }
            if(j == nums) ret.push_back(i);
        }

        return ret;
    }
};