Check whether array A is a permutation.
Solution with poor performance and correctness result https://codility.com/demo/results/trainingR4MCET-EU9/
Correctness 100% score but still poor performance https://codility.com/demo/results/trainingA8B5RA-SSU/
Using Dictionary instead of List for better performance https://codility.com/demo/results/training6UARNG-G44/
Simpler solution https://codility.com/demo/results/training6YUKYH-5QR/