using System;

​

public class MyClass

{

    public static void Main(string[] args)

    {

       int[] test = new int[] {0,2,5,10,11,15,20,21,23,30,45,47,51};

       int result = BinarySearch(test, 0);

       Console.WriteLine(result);

    }

    //returns -1 if key not found, else the index of the key in the array.

    public static int BinarySearch(int[] nums, int key){

        int result = -1;

        //array could be null

        if(nums == null){

            return result;

        }

        int left = 0;

        int right = nums.Length -1;

        while(left <= right){

            int half = (left+right)/2;

            if(nums[half] == key){

                return half;

            }

            if(key < nums[half]){

                right = half-1;

            }

            else{

                left = half+1;

            }

        }

        return result;

    }

}