5. Longest Palindromic Substring --- leetcode

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

原理是manacher算法。从中间向两边扩散，然后维护一个最大回文串的中心和最右端点的下标，当目前的中心在最大半径以内的时候，可以去找对称点，当目前中心在最大半径之外的时候，就要正常去匹配。

class Solution {
public:
    string init(string s)
    {
        string ret;
        ret="%#";
        for(int i=0;i<s.size();i++)
        { 
            ret +=s[i];
            ret +='#';
        };
        return ret;
    }
    string longestPalindrome(string s) {
        string nStr = init(s);
        vector<int> R(nStr.size(),0);
        int mx = 0,id = 0,start=0,maxLen=0;
        for(int i = 1;i<nStr.size();i++)
        {
            if(mx>i)
                R[i] = min(R[2*id-i],mx-i);
            else
                R[i] = 1;
            
            while(nStr[i+R[i]]==nStr[i-R[i]])
                ++R[i];
            if(i+R[i]>mx)
            {
                id = i;
                mx = i+R[i];
            }
            if(R[i]-1>maxLen)
            {
                start = (i-R[i])/2;
                maxLen = R[i]-1;
            }
        }       
        return s.substr(start,maxLen);
    }
};