Substring with Concatenation of All Words

public class Solution {
    public ArrayList<Integer> findSubstring(String S, String[] L) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> results = new ArrayList<Integer>();
        int len = L.length;
        if (S.length() == 0||len == 0) {
            results.add(0);
            return results;
        }

        int M = S.length();
        int N = L[0].length();
        Map<String, Integer> expected = new HashMap<String, Integer>();
        Map<String, Integer> real = new HashMap<String, Integer>();
        for (int j = 0; j < len; j++) {
            if (expected.get(L[j]) == null) {
                expected.put(L[j], 1);
                real.put(L[j], 0);
            } else {
                expected.put(L[j], expected.get(L[j]) + 1);
            }
        }

        for (int i = 0; i <= (M - N * len); i++) {
            int j = 0;
            for (; j < len;) {
                String tmp = S.substring(i + j * N, i + (j + 1) * N);
                if (expected.get(tmp) == null) {
                    break;
                } else {
                    real.put(tmp, real.get(tmp) + 1);
                    if (real.get(tmp) > expected.get(tmp)) {
                        break;
                    }
                    j = j + 1;
                }
            }

            if (j == len) {
                results.add(i);
            }
            for (int m = 0; m < len; m++) {
                real.put(L[m], 0);
            }

        }
        return results;
    }
}