Substring with Concatenation of All Words

题目描述：

You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) ins that is a concatenation of each word inwords exactly once and without any intervening characters.

For example, given:
s: "barfoothefoobarman"
words: ["foo", "bar"]

You should return the indices: [0,9].
(order does not matter).

最笨的办法就是枚举出来所有的子集，然后再从S中得到相应的index、不出意外超时了。

后来用hashtable，创建map<string,integer>，int记录单词的个数，这样也超时了。此时的复杂度是O(m*n)：

后来参考了别人的做法，实在是巧妙。