Longest palindromic Substring length

Question

Below code gives me longest palindromic subsequence length. How can I modify the code to get longest palindromic sub string length?

public static int lp(String str, int i, int j, int ans) {    
    if (i == str.length() || j <= 0)
        return ans;

    if (i > j)
        return ans;

    if (i == j)
        return ans + 1;

    if (str.charAt(i) == str.charAt(j)) {
        int a = lp(str, i + 1, j, ans);
        int b = lp(str, i, j - 1, ans);
        int c = lp(str, i + 1, j - 1, ans + 2);
        ans = Math.max(Math.max(a, b), c);
        return ans;
    } else {
        int a = lp(str, i + 1, j, ans);
        int b = lp(str, i, j - 1, ans);
        ans = Math.max(a, b);
        return ans;
    }
}

Here is an example call:

    String s = "sbsncjss"; 
    // longest subsequence length is 5 and longest substring length is 3
    System.out.println("Ans is " + lp(s, 0, s.length() - 1, 0));

Answer 1

How can I modify the code to get longest palindromic substring length

You then need to exclude the cases where characters are skipped. When your code is about to skip a character, the search can continue, but with ans reset to 0.

So:

public static int lp(String str, int i, int j, int ans) {
    if (i == str.length() || j <= 0)
        return ans;

    if (i > j)
        return ans;

    if (i == j)
        return ans + 1;

    if (str.charAt(i) == str.charAt(j)) {
        int a = lp(str, i + 1, j, 0);   // <-- reset!
        int b = lp(str, i, j - 1, 0);   // <-- reset!
        int c = lp(str, i + 1, j - 1, ans + 2);
        ans = Math.max(Math.max(a, b), c);
        return ans;
    } else {
        int a = lp(str, i + 1, j, 0);   // <-- reset!
        int b = lp(str, i, j - 1, 0);   // <-- reset!
        ans = Math.max(a, b);
        return ans;
    }
}

Be aware that although this works, there are more efficient algorithms for finding the longest palindrome substring . But they are completely different from the algorithm you presented for the longest palindrome subsequence , so it would not be a matter of just making some changes.