【LeetCode】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> list = new ArrayList<Integer>();
int wordLen = L[0].length();
int numOfWords = L.length;
int length = wordLen * numOfWords; // substring length
if (S.length() < length)
return list;

// initialize a hash map to facilitate the word match by word counting
HashMap<String, Integer> map = new HashMap<String, Integer>();
for (String word : L) {
if (!map.containsKey(word)) {
map.put(word, 1);
} else {
map.put(word, map.get(word) + 1);
}
}

for (int i = 0; i <= S.length() - length; i++) {
String substr = S.substring(i, i + length);
HashMap<String, Integer> map2 = (HashMap<String, Integer>) map
.clone();
// partition the substring into the words of equal length
while (true) {
String word = substr.substring(0, wordLen);
if (map2.containsKey(word)) {
int num = map2.get(word) - 1;
// not found: too many occurrences
if (num < 0) {
break;
}
map2.put(word, num);
substr = substr.substring(wordLen);
// found
if (substr.isEmpty()) {
list.add(i);
break;
}
}
// not found: unmatched
else {
break;
}
}
}

return list;
}
}