Find the index of a given permutation in the list of permutations in lexicographic order [duplicate]

Answer 1

Multiply the index of the first digit among the digits in the permutation by (n-1)! and add the index of the remaining permutation.

For example, 2 has index 1, and the index of 13 is 0, so the result is 1 * (3-1)! + 0 = 1 * 2 = 2.