Java: Longest common subsequence

Question

I have following code:

public class LCS1 {

    public static String lcs(String a, String b) {
        String x;
        String y;

        int alen = a.leng         


        

Answer 1

public static String longestSubstring(String str1, String str2) {

    StringBuilder sb = new StringBuilder();
    if (str1 == null || str1.isEmpty() || str2 == null || str2.isEmpty())
        return "";

    // ignore case
    str1 = str1.toLowerCase();
    str2 = str2.toLowerCase();

    // java initializes them already with 0
    int[][] num = new int[str1.length()][str2.length()];
    int maxlen = 0;
    int lastSubsBegin = 0;

    for (int i = 0; i < str1.length(); i++) {
        for (int j = 0; j < str2.length(); j++) {
            if (str1.charAt(i) == str2.charAt(j)) {
                if ((i == 0) || (j == 0))
                    num[i][j] = 1;
                else
                    num[i][j] = 1 + num[i - 1][j - 1];

                if (num[i][j] > maxlen) {
                    maxlen = num[i][j];
                    // generate substring from str1 => i
                    int thisSubsBegin = i - num[i][j] + 1;
                    if (lastSubsBegin == thisSubsBegin) {
                        // if the current LCS is the same as the last time
                        // this block ran
                        sb.append(str1.charAt(i));
                    } else {
                        // this block resets the string builder if a
                        // different LCS is found
                        lastSubsBegin = thisSubsBegin;
                        sb = new StringBuilder();
                        sb.append(str1.substring(lastSubsBegin, i + 1));
                    }
                }
            }
        }
    }

    return sb.toString();
}