public static String lcs(String a, String b) {
    int[][] lengths = new int[a.length()+1][b.length()+1];

    // row 0 and column 0 are initialized to 0 already

    for (int i = 0; i < a.length(); i++)
        for (int j = 0; j < b.length(); j++)
            if (a.charAt(i) == b.charAt(j))
                lengths[i+1][j+1] = lengths[i][j] + 1;
            else
                lengths[i+1][j+1] =
                    Math.max(lengths[i+1][j], lengths[i][j+1]);

    // read the substring out from the matrix
    StringBuffer sb = new StringBuffer();
    for (int x = a.length(), y = b.length();
         x != 0 && y != 0; ) {
        if (lengths[x][y] == lengths[x-1][y])
            x--;
        else if (lengths[x][y] == lengths[x][y-1])
            y--;
        else {
            assert a.charAt(x-1) == b.charAt(y-1);
            sb.append(a.charAt(x-1));
            x--;
            y--;
        }
    }

    return sb.reverse().toString();
}