Manacher算法

public class LongestPalindromicSubstring {
    public String longestPalindrome(String s) {
        StringBuilder sb = new StringBuilder(s); 
        int[] p = new int[2*s.length()+3]; //p数组用来保存每个字符的回文半径
        sb.insert(0, ‘?‘); //这里在开头和结尾设置两个不同的字符，是为了向两边扫描的时候设置字符对比结束，sb最终结果为(?#...#!)
        for(int i=0;i<=s.length();i++){
            sb.insert(2*i+1, ‘#‘);
        }
        sb.insert(2*s.length()+2, ‘!‘);
        int maxL=0,maxId=0,id=0;   //maxL是最长的半径
        int i;
        for(i=1;i<p.length-1;i++){
            if(maxId > i){
                p[i] = min(p[2*id-i], maxId-i);
            }else{
                p[i] = 1;
            }
            while(sb.charAt(i+p[i]) == sb.charAt(i-p[i])){
                p[i]++;
            }
            /*if(p[i] + i > maxId){
                maxId = p[i]+i;
                id = i;
            }*/
            if(p[i] > maxL){
                maxL = p[i];
                id = i;
                maxId = p[i]+i;
            }
        }
        int index = 2*id - maxId + 1;
        index = (index+1)/2-1;
        return s.substring(index, p[id]-1+index);
    }
    
    private int min(int a, int b){
        return a<b?a:b;
    }
    public static void main(String[] args) {
        String s = "ababcbaba";
        String palindrome = new LongestPalindromicSubstring().longestPalindrome(s);
        System.out.println(palindrome);
    }

}