LeetCode Q30: Substring with Concatenation of All Words

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) in s that is a concatenation of each word in wordsexactly 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).

My solution is very straight forward. At each "possible" location, scan entire string. Saying "possible", I mean every wl(word length) position in the string s. So, for example, if wl=5, I will start at 0, 1, 2, 3, 4 entries of string s.

During scan, use a hash table to record current findings. When the word just scanned failed to match, I roll back the scanning pointer to the nearest valid position. In the meantime, the hash table is recovered too.

Code is not optimized and take more than 1000ms to finish, which beat only 8% people in the pool.