答案

public class Solution {
    char special=0;
    public String longestPalindrome(String s) {
        if(s==null||s.length()<=1)
        {
            return s;
        }
        StringBuilder builder=new StringBuilder(2*s.length()+1);
        int i;
        for(i=0;i<s.length();i++)
        {
            builder.append(special);
            builder.append(s.charAt(i));
        }
        builder.append(special);
        int []p=new int[builder.length()];
        int maxPosition=0;
        int central=0;
        int index=0;
        p[0]=1;
        for(i=1;i<builder.length();i++)
        {
            if(maxPosition>=i)
            {
                p[i]=Math.min(p[2*central-i],maxPosition-i+1);
            }
            else
            {
                p[i]=1;
            }
            while(i-p[i]>=0&&i+p[i]<builder.length())
            {
                if(builder.charAt(i-p[i])==builder.charAt(i+p[i]))
                {
                    p[i]++;
                }
                else
                {
                    break;
                }
            }
            if(i+p[i]-1>=maxPosition)
            {
                central=i;
                maxPosition=i+p[i]-1;
            }
            if(p[index]<p[i])
            {
                index=i;
            }
        }
        StringBuilder result=new StringBuilder(p[index]-1);
        for(i=index-p[index]+1;i<index+p[index];i++)
        {
            if(builder.charAt(i)!=special)
            {
                result.append(builder.charAt(i));
            }
        }
        return result.toString();
    }
}