using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        Console.WriteLine(GetMaxIndex(new int[] {1,2,3,4,3,1}));

        Console.WriteLine(GetMaxIndex(new int[] {1,2,3,4,3,2,1,0}));

        Console.WriteLine(GetMaxIndex(new int[] {1,4,6}));

        Console.WriteLine(GetMaxIndex(new int[] {6,3,2,1}));

        Console.WriteLine(GetMaxIndex(new int[] {4,4,3,3,6,2,1}));

    }

    public static int GetMaxIndex(int[] arr){

        if(arr == null || arr.Length == 0)

            return -1;

        return GetMaxIndex(arr, 1, arr.Length);

        //return GetMaxIndexRecursive(arr, 0, arr.Length);

    }

    public static int GetMaxIndex(int[] arr, int left, int right){

        int max = -1;

        while(left < right){

            int mid = left + (right - left)/2;

            if(arr[mid] > arr[mid-1] && (mid+1 == arr.Length || arr[mid] > arr[mid+1])){

                max = mid;

                break; //found

            }else if((mid+1 < arr.Length && arr[mid] < arr[mid+1])) {

                left = mid;

            }else{

                right = mid;

            }

        }

        if(left == right){

            max = left-1;

        }

        return max;

    }

    public static int GetMaxIndexRecursive(int[] arr, int left, int right){

        int mid = left + (right - left)/2;

        if(mid == 0 || mid == arr.Length - 1

          || (arr[mid] > arr[mid-1] && arr[mid] > arr[mid+1])){

            return mid;

        }           

        if(mid+1 < arr.Length && arr[mid] < arr[mid+1]){

            // traverse right

            return GetMaxIndexRecursive(arr, mid, right);

        }else if(mid > 0 && arr[mid-1] > arr[mid]){

            return GetMaxIndexRecursive(arr, left, mid);

        }

        return -1;

    }

}

​