Problem Statement

You’re given a string s and a list of words words, where all words have the same length.

A concatenated substring is formed by joining all the words from any permutation of words — each used exactly once, without any extra characters in between.

For example, if words = ["ab", "cd", "ef"], then valid concatenated strings include "abcdef", "abefcd", "cdabef", "cdefab", "efabcd", and "efcdab". A string like "acdbef" is not valid because it doesn't match any complete permutation of the given words.

Return all starting indices in `s