## Substring with Concatenation of All Words

### 描述

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).

### 代码

// Substring with Concatenation of All Words
// 时间复杂度O(n*m)，空间复杂度O(m)
public class Solution {
public List<Integer> findSubstring(String s, String[] words) {
final int wordLength = words.length();
final int catLength = wordLength * words.length;
List<Integer> result = new ArrayList<>();

if (s.length() < catLength) return result;

HashMap<String, Integer> wordCount = new HashMap<>();

for (String word : words)
wordCount.put(word, wordCount.getOrDefault(word, 0) + 1);

for (int i = 0; i <= s.length() - catLength; ++i) {
HashMap<String, Integer> unused = new HashMap<>(wordCount);

for (int j = i; j < i + catLength; j += wordLength) {
final String key = s.substring(j, j + wordLength);
final int pos = unused.getOrDefault(key, -1);

if (pos == -1 || pos == 0) break;

unused.put(key, pos - 1);
if (pos - 1 == 0) unused.remove(key);
}