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.
S: "barfoothefoobarman"
L: ["foo", "bar"]
You should return the indices: [0,9].
(order does not matter).

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