HackerRank - Stryker world code sprint - study code - Array.sort call using compare function - how to define

PointFiltering_studyCode_A.cs

using System;
using System.Collections.Generic;
using System.IO;

class PointF3D{
    public int Id{get; set;}
    public double X{get; set;}
    public double Y{get; set;}
    public double Z{get; set;}
    
    public override string ToString(){
        return string.Format("{0} = ({1:F3},{2:F3},{3:F3})", Id, X, Y, Z);
    }
}

class Solution {
    static void Main(String[] args) {
        var nb = Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
        int n = nb[0];
        int b = nb[1];
        
        Dictionary<int, PointF3D> bucket = new Dictionary<int, PointF3D>(b);
        PointF3D[] points = new PointF3D[n];
        for(int i = 0; i < n; i++){
            var line = Console.ReadLine().Split(' ');
            points[i] = new PointF3D{
                Id = int.Parse(line[0]), 
                X = double.Parse(line[1]),
                Y = double.Parse(line[2]),
                Z = double.Parse(line[3])
            };
        }
        Array.Sort(points, CompareByZ);
        int index = b; 
        for(int i = 0; i < b; i++){
            bucket.Add(points[i].Id, points[i]);
        }
        
        while(true){
            string line = Console.ReadLine();
            if(string.IsNullOrWhiteSpace(line)){
                break;
            }
            var query = line.Split(' ');
            var k = int.Parse(query[1]);
            if(query[0] == "f" || query[0] == "F"){
                if(bucket.ContainsKey(k)){
                    Console.WriteLine(bucket[k]);
                }
                else{
                    Console.WriteLine("Point doesn't exist in the bucket.");
                }
            }
            else{
                if(bucket.ContainsKey(k)){
                    if(index >= n){
                        Console.WriteLine("No more points can be deleted.");
                    }
                    else{
                        bucket.Remove(k);
                        var p = points[index];
                        index++;
                        bucket.Add(p.Id, p);
                        Console.WriteLine("Point id {0} removed.", k);
                    }
                }
                else{
                    Console.WriteLine("Point doesn't exist in the bucket.");
                }
            }
        }
    }
    
    static int CompareByZ(PointF3D p1, PointF3D p2){
        return p2.Z.CompareTo(p1.Z);
    }
}