Array value equal index - June 8 practice, two issues, first time complexity is O(n), not O(logn); And the smallest index is not found. Edge case is handled properly.

ArrayValueEqualIndex_June8_practice.cs

using System;

class Solution
{
    static int indexEqualsValueSearch(int[] arr) 
    {
      // your code goes here
      if(arr == null || arr.Length == 0)
      {
        return -1; 
      }
      
      int length = arr.Length; 
      
      for(int i = 0; i < length; i++)
      {
        arr[i] = arr[i] - i; 
      }
      
      // binary search 0 on arr 
      
      var index = BinarySearch(arr, 0, length); 
      return index; 
    }
  
    // -8, -1, 0, 3, start = 0, end = 3
    // 100, 0, end = 0
    public static int BinarySearch(int[] numbers, int start, int end)
    {
      if( start > end)
      {
        return -1; 
      }
      
      while(start <= end) // 0 <= 3; 2 <=3
      {
        int middle = start + ( end - start) / 2; // 1, 2
        int value = numbers[middle]; // -1, 0
        
        bool isSearchValue = value == 0; // true
        
        if(isSearchValue)
        {
          return middle; // 2
        }
        else if( value > 0 )
        {
          end = middle - 1; // -1 
        }
        else
        {
          start = middle + 1; // 2
        }        
      }
      
      return -1; 
    }

    static void Main(string[] args)
    {
        var testcase = new int[]{-8, 0, 2,5}; 
        Console.WriteLine(indexEqualsValueSearch(testcase)); 
      
        var testcase2 = new int[]{-8, 1, 2,5}; 
        Console.WriteLine("second case is "+ indexEqualsValueSearch(testcase2)); 
    }
}

// -8, 0, 2, 5  -> sorted array, assuming ascending order, gap element >= 1
// distinct integers 
// 0, 1,  2, 3  -> sorted array, gap = 1
// gap element >=1 - 1 => gap >= 0 
// -8, -1, 0, 2 <-- see if there is an element which has value zero
// -8, 0, 2, 3, 5
// 0, 1,  2, 3, 4
// two elements = 
// return first one -> O(n) -> O(logn)  
// use the same array taken away index value, and then binary search the array