How can I find the length of lcs between two large lines

Question

I wrote the following code in C#order to get the length of the longest common subsequence of two texts specified during use, but does not work with large lines. Could you 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. Thus, this leads to the fact that the program performs 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 on 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);
        }
    }
}

: http://www.snippetsource.net/Snippet/75/longest-common-substring