Longest Palindrome in a String

class Solution{
static String longestPalin(String S){

int start=0,end=0;
String result="";
int prevLength=0;
for(int i=0;i<S.length();i++){ 
    int len1=expandAround(S,i,i);
    int len2=expandAround(S,i,i+1);
    int len=Math.max(len1,len2);
    if(len>end-start){
start= i-((len-1)/2);
end=i+(len/2);
if(len !=prevLength){
result= S.substring(start,end+1);
prevLength=len;
}
}
}
if(start==end){
return String.valueOf(S.charAt(0));
}
//return s.substring(start,end+1);
return result;
}


public static int expandAround(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;
}


}