using System;

public class Program {

  public static bool IsPerfect(int[] arr) {

    // In public method we must be ready for any input; null included 

    if (arr == null)

      return false; // or throw new ArgumentNullException(nameof(arr));

​

    // Do we have a valid array?

    for (int i = 0; i < arr.Length; ++i)

      if (arr[i] < 0 || arr[i] >= arr.Length)

        return false; 

​

    // Note the condition: we read the next item - arr[i] -

    // if and only if i is within range

    for (int i = 0; i >= 0 && i < arr.Length; ) 

      (i, arr[i]) = (arr[i], -1);

​

    // We have marked all that we could. 

    // Do we have unmarked items? 

    for (int i = 0; i < arr.Length; ++i)

      if (arr[i] != -1) // ... yes, we have and thus 

        return false;   // arr is not perfect

    return true; 

  }

  public static void Main() {

    int[] arr = { 3, 0, 1, 4, 2 };

    Console.Write(IsPerfect(arr) ? "Perfect" : "NOT Perfect");  

  }

}