Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

Sol:

class Solution {
    public String longestPalindrome(String s) {
        
        if(s==null || s.length()<1) return "";
        int start, end;
        start=end=0;
        for(int i=0; i<s.length(); i++) {
            
            int len1=expandAroundCenter(s, i, i);
            int len2=expandAroundCenter(s, i, i+1);
            int len = Math.max(len1, len2);
            if(len>(end-start)){
                start=i-(len-1)/2;
                end=i+len/2;
            }
        }
        
        return s.substring(start, end+1);
    }
    
private int expandAroundCenter(String s, int i, int j) {
       while(i>=0 && j<s.length() && s.charAt(i)==s.charAt(j)) {
            i--;
            j++;
        }
        return j-i-1;
    }
}