Pair with specific difference - linear two pointers

PairWithSpecificDiff_Linear.cs

using System;
using System.Collections.Generic; 

class Solution
{
    public static int[,] FindPairsWithGivenDifference(int[] arr, int k) // [0, -1, -2, 2, 1], k = 1
    {
      if(arr == null || arr.Length <= 1 || k < 0) // false 
      {
        return new int[0,0]; 
      }
      
      int length = arr.Length;  // 5
      
      Array.Sort(arr); // -2, -1, 0, 1, 2
      
      var pair = new List<int[]>(); 
      
      int front = 1; 
      int back  = 0;       
     
      while(front < length && back < length )   // [1, 5, 7, 11]
      {
        var bigger  = arr[front]; 
        var smaller = arr[back];
        
        var diff = bigger - smaller; 
        var found = diff == k; 
        if(found)
        {
          // add to the pair
          pair.Add(new int[]{bigger, smaller}); 
          //
          back++;
        }
        else if(diff < k )
        {
          front++; 
        }
        else 
        {
          back++; 
        }
      }
      
        
      // convert to the two dimension array
      var pairLength = pair.Count; 
      
      var pairArray = new int[pairLength, 2]; 
      
      for(int i = 0; i < pairLength; i++)
      {
        pairArray[i, 0] = pair[i][0]; 
        pairArray[i, 1] = pair[i][1]; 
      }
      
      return pairArray; 
    }

    private static bool binarySearch(int[] numbers, int start, int end, int search ) // 1, 4, 1
    {
      while(start <= end)    // true
      {
        int middle = start + (end - start)/ 2; // 2
        var middleValue = numbers[middle]; // 0
        
        var found = middleValue == search; // false 
        if(found)
        {
          return true;
        }
        else if(middleValue < search) // 0 < 1
        {
          start = middle + 1; // 3
        }
        else 
        {
          end = middle - 1; 
        }        
      }
      
      return false; 
    }
  
    static void Main(string[] args)
    {
        var pair = FindPairsWithGivenDifference(new int[]{0, -1, -2, 2, 1}, 1); 
      
        var rows = pair.GetLength(0);
      
       for(int i = 0; i < rows; i++)
       {
         Console.WriteLine(pair[i, 0] + "," + pair[i, 1]); 
       }
    }
}

/*
[0, -1, -2, 2, 1], k = 1  x - y = k , k = 1, x > y
O(n^2)

sort the array -  O(nlogn)
[-2, -1, 0, 1, 2]  -> all possible pairs -2, need to find -2 + 1 = -1, binary search  nlogn

O(nlogn)

extra space 
scan the array, in the same time, put past integer in hashset, 
-2, try to find -3, -2 hashset
-1, -2 in hashset, [-1, -2] result list, 
[-1] in hashset
-> O(n), space O(n)
*/