You may not a beginner anymore, but I think the complexity of this is O(n^2) where n is the len of the secret.

I use "triplets * 2" have passed.
but i don't know why use it.

letters is a list of all the different letters in the string, and the for loop is just repeatedly looking a the triplets
eg. if triplets=[[1,2],[3,4]] then letters is a list containing 1,2,3,4 in any order, and t will be [1,2] and [3,4], 4 times each