how can i find lcs length between two large strings

Question

I've written the following code in C# for obtaining the length of longest common subsequence of two texts given by use, but it doesn't work with large strings. Could you please help me. I'm really confused.

public Form1()
{
    InitializeComponent();
}

public int lcs(char[] s1, char[] s2, int s1size, int s2size)
{
    if (s1size == 0 || s2size == 0)
    {
        return 0;
    }
    else
    {
        if (s1[s1size - 1] == s2[s2size - 1])
        {
            return (lcs(s1, s2, s1size - 1, s2size - 1) + 1);
        }
        else
        {
            int x = lcs(s1, s2, s1size, s2size - 1);
            int y = lcs(s1, s2, s1size - 1, s2size);
            if (x > y)
            {
                return x;
            }
            else
                return y;
        }
    }
}

private void button1_Click(object sender, EventArgs e)
{
    string st1 = textBox2.Text.Trim(' ');
    string st2 = textBox3.Text.Trim(' ');

    char[] a = st1.ToCharArray();
    char[] b = st2.ToCharArray();

    int s1 = a.Length;
    int s2 = b.Length;

    textBox1.Text = lcs(a, b, s1, s2).ToString(); 
}

Answer 1

Here you are using the Recursion method. So it leads the program to occur performance problems as you mentioned.

Instead of recursion, use the dynamic programming approach.

Here is the C# Code.

public static void LCS(char[] str1, char[] str2)
    {
        int[,] l = new int[str1.Length, str2.Length];
        int lcs = -1;
        string substr = string.Empty;
        int end = -1;

        for (int i = 0; i < str1.Length; i++)
        {
            for (int j = 0; j < str2.Length; j++)
            {
                if (str1[i] == str2[j])
                {
                    if (i == 0 || j == 0)
                    {
                        l[i, j] = 1;
                    }
                    else
                        l[i, j] = l[i - 1, j - 1] + 1;
                    if (l[i, j] > lcs)
                    {
                        lcs = l[i, j];
                        end = i;
                    }

                }
                else
                    l[i, j] = 0;
            }
        }

        for (int i = end - lcs + 1; i <= end; i++)
        {
            substr += str1[i];
        }

        Console.WriteLine("Longest Common SubString Length = {0}, Longest Common Substring = {1}", lcs, substr);
    } 

Answer 2

Here is a solution how to find the longest common substring in C#:

public static string GetLongestCommonSubstring(params string[] strings)
{
    var commonSubstrings = new HashSet<string>(strings[0].GetSubstrings());
    foreach (string str in strings.Skip(1))
    {
        commonSubstrings.IntersectWith(str.GetSubstrings());
        if (commonSubstrings.Count == 0)
            return string.Empty;
    }

    return commonSubstrings.OrderByDescending(s => s.Length).DefaultIfEmpty(string.Empty).First();
}

private static IEnumerable<string> GetSubstrings(this string str)
{
    for (int c = 0; c < str.Length - 1; c++)
    {
        for (int cc = 1; c + cc <= str.Length; cc++)
        {
            yield return str.Substring(c, cc);
        }
    }
}

I found it here: http://www.snippetsource.net/Snippet/75/longest-common-substring