The Last-to-First array, denoted LastToFirst(i), answers the following question: given a symbol at position i in the last column of a Burrow-Wheeler matrix, what is its position in first column?
Implement the last_to_first_mapping method which takes a string containing a '$' and an index in the last column, returning the index of that symbol in the first column.
>>> last_to_first_mapping('T$GACCA', 3) 1 >>> last_to_first_mapping('CAAGCGGTTAGCAACTGCATGCCACCAGGGTTTCTTCCATGAGTAACTACTTTCTACGTACTGGCCCGCTGGGCCGGGTAGAGTCGCTAAACCAGCCTGGGACCGATGTTGATCCGCCCACCCAAAACTGTGTACCGTGGCACCGAACTGGCGAACAAAAGGACATCGCTTACCCCTGAACATTGACCACGGCATGGCCATTCTCCCTGAATACGCGAAAACCTATTTGACGGGACCTAACAAGCACATACTCGTACTTATCGAGTCCTCGGCTAGTGACTTTTGTCTTCTAGATCGGGGTGTCGGGGGATGGGGGATTTCGGTATGCGGCACCTCGGCGTGCTTGAACTGTAGGAACCACAGATTCTCCAAACTAAATTGTTATCCTTTTACAGACGGGACACACGTGTCCAGGAGTAAATCGGGTCACTCAGCACGCATTTCGCTATCGTATTTCCCCGAATCCTTAACAAAACTGAGGTAGTAGGGAGCCCTCTGTCGTAGATTGATAACAGCGTCAGTCGAGTGCCCGAGATGTGCACCATGACGCTTTTGTGATATTGACTAATGGTGGCGGTACAGGGCCATGACCTCTATCCTCTAAATACGCGATACTAATGAAGGTTCCTCGGTGGGATGGTCGTGACTTTGCAACCGTTCCTCGCCAAGGGGGGACGAAGCTCCAGTGTGATTCTTACTTCTAACAGCCGGGTGTCGGACGCCTACCGGTCCTCTTAGTGGATGCGTGACATGTGCTTATTTACGGTCGACTGATAGTAAGGCCGGACTTGGTTATCAATTCACAACAGTTTGGTGTACGCCTTGTACAGAGCTGTTTAGAAATAGTGACAGTTTCGAACCATATAGCGACTGGCTAACCTGAATCAATAGATGCAAAAGGACGACTACTACTGATATGAGTGGGCTTTAAGTACACTGAGAG$', 235) 304