Longest Palindromic Substring

多方参考后写的一个java版本的，通过oj。

public String longestPalindrome(String s) {
    	// $#a#b#c#b#@ abcb notes: p[i] = right move + 1. So right doesn't count.
        int len = s.length();
        if(len < 2) return s;
        char[] ss = new char[len * 2 + 3];
        ss[0]= '$'; ss[len*2+2] = '@';
        
        for(int i = 1; i < len * 2 + 2; i ++){
        	if(i %2 == 0){
        		ss[i] = s.charAt(i/2 - 1);
        	} 
        	else 
        		ss[i] = '#';
        }
        
    //    System.out.println("Array is : " + Arrays.toString(ss));
        
        int[] p = new int[ss.length];
        int center = 0, right = 0;
        for(int i = 1; i < ss.length - 1; i ++){
        //	
        	int mirror = 2 * center - i;
        	if(right > i) p[i] = Math.min(p[mirror], right - i);
        	
        	
        	while(ss[i-p[i]] == ss[i+p[i]]) p[i] ++;
        	if(i + p[i] > right){
        		center = i;
        		right = i + p[i];
        	}
        }
                
        //find answer
        int max = 0;
        for(int i = 1; i < p.length -1; i ++){
        	if(p[i] > max){
        		max = p[i];
        		center = i;
        	}
        }
        return s.substring((center - p[center])/2, (center + p[center])/2 -1);
        	
    }