Question

I’m trying to find a subpalindrome in Java. But the below code doesn’t provide the correct output.

public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    in.useDelimiter("\n");
    String text = in.next();
    int start = 0;
    int end = 0;

    int i = 0;
    int j = 0;
    while(i <= text.length())
    {
        j = i+1;
        while(j < text.length())
        {
            if(text.substring(i, j).equals(reverse(text.substring(i, j))))
            {
                if(j-i > end-start)
                {
                    start = i; end = j;
                }   
            }
            j++;
        }
        i++;
    }
    System.out.println(start + " : " + end);
}
static String reverse(String s)
{
    return new StringBuffer(s).reverse().toString();
}

Sample Output:

apros tda adda
7 : 12
after the ostso
11 : 14
att feref
5 : 8

All of the above are wrong.

PS: I’m aware that this is not an efficient algorithm.

Answer 1

Finally got it working.

public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    in.useDelimiter("\n");
    String text = in.next();

    int start = 0;
    int end = 0;

    int i = 0;
    int j = 0;
    while(i < text.length())
    {
        j = i;
        while(j <= text.length())
        {
            if(text.substring(i, j).equals(reverse(text.substring(i, j))))
            {
                if(j-i > 1 && j-i > end-start)
                {
                    start = i; 
                    end = j;
                }   
            }
            j++;
        }
        i++;
    }
    System.out.println(start + " : " + (end-1));
}
static String reverse(String s)
{
    return new StringBuffer(s).reverse().toString();
}

Here is a more optimized version. This is based on this

public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    in.useDelimiter("\n");
    char[] text = in.next().toCharArray();

    int start = 0;
    int end = 0;
    ArrayList<Integer[]> list = new ArrayList<Integer[]>();
    for( ;start<text.length; start++)
    {
        for(end=start; end<=start+1; end++)
        {
            list.add(grow(text, start, end));
        }
    }
    for(Integer[] v: list)
        System.out.println(v[0] +" : "+v[1]);
    findmax(list);
}
static Integer[] grow(char[] text, int start, int end)
{
    while(start>0 && end < text.length && text[start-1]==text[end])
    {
        start--;
        end++;
    }
    return new Integer[]{start, end};
}
static void findmax(ArrayList<Integer[]> list)
{
    int i=0; int j=0;
    for(Integer[] v: list)
    {
        if(v[1]-v[0]>j-i)
        {
            i = v[0];
            j = v[1];
        }
    }
    System.out.println(i+" "+j);
}