Longest Palindromic Substring

class Solution {
  public String longestPalindrome(String s) {
    if(s==null || s.length()<1) return"";
    int start=0;
    int end=0;
    for(int i=0;i<s.length();i++){
      int len1=expfromiddle(s,i,i);
      int len2=expfromiddle(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);
  }
  public int expfromiddle(String s, int left, int right){
    if(s==null||left>right) return 0;
    while(left>=0 && right<s.length()&&s.charAt(left)==s.charAt(right)){
      left--;
      right++;
    }
    return right-left-1;
  }
}