Given an array of integers sorted in descending order and in the range 1 through n, write some code to find the index of the first instance of a specific number and the total number of instances that are in the array.

FindIndexAndCount.cs

using System;

public class Program
{
	public static void Main()
	{
		var ans = GetIndexAndCount(new int[] {10, 9, 9, 8, 8, 7, 7, 6, 6, 5, 5, 4, 4, 4, 3, 3, 3, 3}, 9);
		
		Console.Write(ans[0] + " " + ans[1]);
	}
	
	public static int[] GetIndexAndCount(int[] nums, int target) {
        if (nums.Length == 0) { return new int[] {-1, -1}; }
        
        int start = 0, end = nums.Length - 1, mid = 0;
        
        while (start <= end) {
            mid = start + (end - start) / 2;
            
            if (nums[mid] == target) {
                return GetIndexAndCountUtil(mid, nums, target);
            } else if (nums[mid] > target) {
                start = mid + 1;
            } else {
                end = mid - 1;
            }
        }// end of while
        
        return new int[] {-1, -1};
    }
    
    public static int[] GetIndexAndCountUtil(int index, int[] nums, int target) {
        int start = index, end = index;
        bool update = true;
        
        while (update) {
            update = false;
            if (start > 0 && nums[start - 1] == target) {
                start--;
                update = true;
            }
            if (end < nums.Length - 1 && nums[end + 1] == target) {
                end++;
                update = true;
            }            
        }// end of while
        
        return new int[] {start, (end - start) + 1};
    }
}