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).

public class Solution {
    public ArrayList<Integer> findSubstring(String S, String[] L) {
        ArrayList<Integer> res = new ArrayList<Integer> ();
        int len1 = S.length();
        int len2 = L.length;
        if(len2<=0||len1<=0) return res;
        int wordLen = L[0].length();
        HashMap<String,Integer> ex = new HashMap<String,Integer>();
        HashMap<String,Integer> app = new HashMap<String,Integer>();
        for(int i=0;i<len2;i++){
            if(ex.get(L[i])==null) {
                ex.put(L[i],1);
                app.put(L[i],0);   
            }
            else ex.put(L[i],ex.get(L[i])+1);
        }
        for(int i=0;i<=len1-len2*wordLen;i++){
            int start = i;
            String temp = S.substring(start,start+wordLen);
            while(ex.get(temp)!=null && ex.get(temp)>app.get(temp)){
                app.put(temp,app.get(temp)+1);
                start +=wordLen;
                if(start+wordLen>len1)break;
                temp = S.substring(start,start+wordLen);
            }
            if(start-i==wordLen*len2)
                res.add(i);
            for(int j=0;j<len2;j++){
                app.put(L[j],0);
            }
        }
        return res;
    }
}