[LeetCode] Longest PalinDromic Substring

Thought: 
    It is not difficult to come up with the idea to populate every character and expand along its two sides. This method will get a O(n^2) time complexity.
    There surprisingly exists a very nontrivial algorithm here whose time complexity is O(n)! Manacher’s Algorithm

Code:
public class Solution {
    public String longestPalindrome(String s) {
        String input = preProcess(s);
        int len = input.length();
        int[] P = new int[len];
        int center = 0, right = 0;
        for( int i = 1; i < len - 1; i++ ){
            int j = 2 * center - i;
            P[i] = right > i? Math.min( right - i, P[j] ): 0;
            while( input.charAt(i + P[i] + 1) == input.charAt(i - P[i] - 1) ){
                P[i]++;
            }
            if( i + P[i] > right){
                right = i + P[i];
                center = i;
            }
        }        
        int resultc = 0, resultl = 0;
        for( int i = 0 ; i < len; i++ ){
            if(P[i]>resultl){
                resultc = i;
                resultl = P[i];
            }
        }        
        return s.substring( (resultc - resultl - 1)/2, (resultc + resultl)/2 - 0 );
    }
    public String preProcess(String s){
        String temp = "!#";
        for( int i = 0; i < s.length(); i++ ){
            temp = temp + s.substring(i, i + 1);
            temp = temp + "#";
        }
        temp = temp + "@";
        return temp;
    }
}