C# implementation of KMP algorithm

Program.cs

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

/*
Reference: http://www.geeksforgeeks.org/searching-for-patterns-set-2-kmp-algorithm/
*/

namespace LeetCodeTest
{
    class KMP
    {
        public List<int> KMPSearch(string text, string pattern)
        {
            int N = text.Length;
            int M = pattern.Length;
            
            if(N < M) return -1;
            if(N == M && text == pattern) return 0;
            if(M == 0) return 0;
            
            int[] lpsArray = new int[M];
            List<int> matchedIndex = new List<int>();

            LongestPrefixSuffix(pattern, ref lpsArray);

            int i = 0, j = 0;
            while (i < N)
            {
                if (text[i] == pattern[j])
                {
                    i++;
                    j++;
                }

                // match found at i-j
                if (j == M)
                {
                    matchedIndex.Add(i - j);
                    Console.WriteLine((i-j).ToString());
                    j = lpsArray[j - 1];
                }
                else if (i < N && text[i] != pattern[j])
                {
                    if (j != 0)
                    {
                        j = lpsArray[j - 1];
                    }
                    else
                    {
                        i++;
                    }
                }
            }

            return matchedIndex;
        }

        public void LongestPrefixSuffix(string pattern, ref int[] lpsArray)
        {
            int M = pattern.Length;
            int len = 0;
            lpsArray[0] = 0;
            int i = 1;

            while (i < M)
            {
                if (pattern[i] == pattern[len])
                {
                    len++;
                    lpsArray[i] = len;
                    i++;
                }
                else
                {
                    if (len == 0)
                    {
                        lpsArray[i] = 0;
                        i++;
                    }
                    else
                    {
                        len = lpsArray[len - 1];
                    }
                }
            }
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            KMP kmp = new KMP();
            kmp.KMPSearch("bangladesh", "glade");
            kmp.KMPSearch("ABABDABACDABABCABAB", "ABABCABAB");
        }
    }
}