Finding the index of a given permutation

Question

I\'m reading the numbers 0, 1, ..., (N - 1) one by one in some order. My goal is to find the lexicography index of this given permutation, using only O(1)

Answer 1

There is a java solution to this problem on geekviewpoint. It has a good explanation for why it's true and the code is easy to follow. http://www.geekviewpoint.com/java/numbers/permutation_index. It also has a unit test that runs the code with different inputs.