Modified binary search - 30 minutes practice - I got advise to work on middle instead of start, the binary search is to search the middle.

modifiedBinarySearch_practice.cs

using System;

class Solution
{
    public static int IndexEqualsValueSearch(int[] arr)
    {
      // your code goes here
      if(arr == null || arr.Length == 0)
      {
        return -1; 
      }
      
      // assume that array is not empty 
      var start = 0; 
      var end = arr.Length - 1; 
      while(start <= end)
      {                
        var middle = start + (end - start)/ 2; 
        
        var middleValue = arr[middle] - middle; 
        bool found = middleValue == 0; 
        if(found)
        {
          return middle; 
        }
        
        if(middleValue < 0)
        {
          start = middle + 1; 
        }
        else // > 0 
        {
          end = middle - 1; 
        }
      }
      
      return -1; 
    }

    static void Main(string[] args)
    {

    }
}