Write a function that merges an array of already sorted arrays, producing one large, still sorted array. (included a C# integer array merge sort method)

CodingInterviewPractice_2012_12_24.cs

/****************************************************************
    Adam Lum
    12/28/2012

    Coding for Interviews on Merge Sort
    http://codingforinterviews.com

    Write a function that merges an array of already sorted arrays, 
    producing one large, still sorted array.
  
   (included a C# integer array merge sort method)
    
*******************************************************************/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CodingInterviewPractice_2012_12_24
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] firstSorted       = { 0, 5, 8, 9 };
            int[] secondSorted      = { 1, 2, 7 };
            int[] thirdSorted       = { 10 };
            int[][] arrayOfSorted   = { firstSorted, secondSorted, thirdSorted};

            int[] largeSorted = mergeArrayOfSortedArrays(arrayOfSorted);

            for(int i = 0; i < largeSorted.Length; i++)
            {
                if (i > 0)
                {
                    Console.Write(", ");
                }
                Console.Write(largeSorted[i]);
            }
        }

        static int[] mergeArrayOfSortedArrays(int[][] _arrayOfSorted)
        {
            int[] returnArray = new int[0];

            for (int i = 0; i < _arrayOfSorted.Length; i++)
            {
                returnArray = merge(returnArray, _arrayOfSorted[i]);
            }

            return returnArray;
        }

        static int[] mergeSort(int[] _unsorted)
        {
            if (_unsorted.Length > 1)
            {
                int[] first, second;
                if (_unsorted.Length % 2 == 0)
                {
                    first   = new int[_unsorted.Length / 2];
                    second  = new int[_unsorted.Length / 2]; 
                }
                else
                {
                    first   = new int[_unsorted.Length / 2];
                    second  = new int[(_unsorted.Length / 2) + 1];
                }

                Array.Copy(_unsorted, first, first.Length);
                Array.Copy(_unsorted, first.Length, second, 0, second.Length);

                return merge(mergeSort(first), mergeSort(second));
            }
            else
            {
                return _unsorted;
            }
        }

        static int[] merge(int[] _first, int[] _second)
        {
            int[] sorted = new int[_first.Length + _second.Length];
            int firstIndex = 0, secondIndex = 0;
            
            for (int i = 0; i < sorted.Length; i++)
            {
                if (firstIndex == _first.Length)
                {
                    sorted[i] = _second[secondIndex];
                    secondIndex++;
                }
                else if (secondIndex == _second.Length)
                {
                    sorted[i] = _first[firstIndex];
                    firstIndex++;
                }
                else if (_first[firstIndex] < _second[secondIndex])
                {
                    sorted[i] = _first[firstIndex];
                    firstIndex++;
                }
                else
                {
                    sorted[i] = _second[secondIndex];
                    secondIndex++;
                }
            }

            return sorted;
        }
    }
}