LCS - 1

String str1 = "GTCGTTCG";
String str2 = "ACCGGTCGAGTG";

int[,] l = new int[str1.Length, str2.Length]; // String 1 length and string 2      length storing it in a 2-dimensional array
int lcs = -1;
string substr = string.Empty;
int end = -1;

for (int i = 0; i <str1.Length ; i++) // Looping based on string1 length
{               
  for (int j = 0; j < str2.Length; j++) // Looping based on string2 Length
  {                 
    if (str1[i] == str2[j]) // if match found
    {
      if (i == 0 || j == 0)  // i is first element or j is first elemnt then array [i,j] = 1
      {
        l[i, j] = 1;
      }
      else
      { 
        l[i, j] = l[i - 1, j - 1] + 1; // fetch the upper value and increment by 1
      }

      if (l[i, j] > lcs)
      {
        lcs = l[i, j]; // store lcs value - how many time lcs is found
        end = i; // index on longest continuous string
      }

    }
    else // if match not found store zero initialze the array value by zero
    {
      l[i, j] = 0;
    }
}