using System;

public class Program

{

    public static void Main()

    {

        // Set permutation matrix here:

        var p = new int[]{3, 1, 2};

        // Inverse permutation matrix is the transposed permutation matrix

        var pt = PTransposed(p);

        Console.WriteLine($"P: [{string.Join(',', p)}]");

        Console.WriteLine($"P^-1: [{string.Join(',', pt)}]");

    }

    public static int[] PTransposed(int[] p)

    {

        var pt = new int[p.Length];

        // Original p[] is a mapping from index -> value

        // Transposed p == Inverse p == mapping from value to index (possible because P is a NxN Matrix)

        // Swap indices with values

        // (C# has 0-based array indices)

        for (var i = 0; i < p.Length; i++)

        {

            var ptIndex = p[i] - 1;

            var ptValue = i+1;

            pt[ptIndex] = ptValue;

        }

        return pt;

    }

}