Substring with Concatenation of All Words.java

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).

public class Solution {
    public ArrayList<Integer> findSubstring(String S, String[] L) {
        int slen = S.length();
        int m = L.length;
        ArrayList<Integer> ret = new ArrayList<Integer>();
        if (m == 0) return ret;
        int n = L[0].length();
        HashMap<String, Integer> smap = new HashMap<String, Integer>();
        for (String s : L) {
            if (smap.containsKey(s)) {
        	smap.put(s, smap.get(s).intValue() + 1);
            } else {
        	smap.put(s, 1);
            }
        }
        if (m * n > slen) return ret;
        int start = 0, end = m * n - 1;
        int index = 0;
        while (end < slen) {
            index = check(S, start, end, m, n, L, smap);
            if (index >= 0) ret.add(index);
            start++;
            end++;
        }
        return ret;
    }
	
    private int check(String s, int start, int end, int m, int n, String[] L, HashMap<String, Integer> smap) {
	HashMap<String, Integer> exitMap = new HashMap<String, Integer>();
	int sidx = start;
	while(sidx <= end) {
	    String k = s.substring(sidx, sidx + n);
	    if (smap.containsKey(k)) {
		if (exitMap.containsKey(k)) {
		    Integer i = exitMap.get(k);
		    if (i == smap.get(k)) return -1;
		    else {
			exitMap.put(k, i + 1);
		    }
		} else {
		    exitMap.put(k, 1);
		}
		sidx += n;
	    } else {
		return -1;
	    }
	}
	return start;
    }
}