[LeetCode] Substring with Concatenation of All Words

Problem

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).

Algorithm

Use a hash table to count the occurrence of strings in list L. Scan S from left to right. Start from any index in S, if the substring S.substring(index, index + L.length()) is a concatenation of words in L, we are done. 

This problem is smilier to Longest Substring without Repeating Characters, which maintained a window and using a hash table to count the occurrence of characters. 

Code