GetOrderedNumbersCount.cs

/*
mkdir OrderedNumbers
cd ./OrderedNumbers 
dotnet new xunit
*/ 

using System;
using System.Collections.Generic;
using System.Linq;
using Xunit;

namespace OrderedNumbers
{
    public class OrderedNumbersTests
    {
        public static int GetOrderedNumbersCount(int[] numbers, int n)
        {
            if (n == 0 || n == 1)
            {
                return n;
            }

            var lefts = new List<int>();
            var leftMax = numbers[0];
            lefts.Add(numbers[0]);

            for (int i = 1; i < n; i++)
            {
                if (numbers[i] > leftMax)
                {
                    lefts.Add(numbers[i]);
                    leftMax = numbers[i];
                }
            }

            var rights = new List<int>();
            var rightMin = numbers[n - 1];
            rights.Add(numbers[n - 1]);

            for (int i = n - 2; i >= 0; i--)
            {
                if (numbers[i] < rightMin)
                {
                    rights.Add(numbers[i]);
                    rightMin = numbers[i];
                }
            }

            var count = 0;
            var hashSetLefts = new HashSet<int>(lefts);
            foreach (var right in rights)
            {
                if (hashSetLefts.Contains(right))
                {
                    count++;
                }
            }
            //var count = rights.Intersect(lefts).Count();
            return count;
        }

        [Fact]
        public static void ForBasic_Pass()
        {
            // Arrange
            var numbers = new int[] { 1, 2, 3 };

            // Act
            var actual = GetOrderedNumbersCount(numbers, numbers.Length);

            // Assert
            Assert.Equal(3, actual);
        }

        [Fact]
        public static void ForSingle_Pass()
        {
            // Arrange
            var numbers = new int[] { 1 };

            // Act
            var actual = GetOrderedNumbersCount(numbers, numbers.Length);

            // Assert
            Assert.Equal(1, actual);
        }

        [Fact]
        public static void ForEmpty_Pass()
        {
            // Arrange
            var numbers = new int[] { };

            // Act
            var actual = GetOrderedNumbersCount(numbers, numbers.Length);

            // Assert
            Assert.Equal(0, actual);
        }

        [Fact]
        public static void ForReverseArray_Pass()
        {
            // Arrange
            var numbers = new int[] { 5, 4, 3, 2, 1 };

            // Act
            var actual = GetOrderedNumbersCount(numbers, numbers.Length);

            // Assert
            Assert.Equal(0, actual);
        }

        [Fact]
        public static void ForUnOrderedArray_Pass()
        {
            // Arrange
            var numbers = new int[] { 54, 45, 37, 22, 10, 69, 95, 82, 71 };

            // Act
            var actual = GetOrderedNumbersCount(numbers, numbers.Length);

            // Assert
            Assert.Equal(1, actual);
        }
    }
}