leetcode 214: Shortest Palindrome

Use KMP algorithm. First, get the reverse of the string s called new_s, and add s, a special character ('#') and new_s together. Word out the next array on this new_s. Then next[len] will give the longest same prefix and postfix, in another word, longest palindrome.

I learned the KMP algorithm from http://blog.youkuaiyun.com/v_july_v/article/details/7041827.

class Solution {
public:
    string shortestPalindrome(string s) {
        string new_s=s;
        reverse(new_s.begin(),new_s.end());
        new_s=s+'#'+new_s;//to avoid error if s is empty or s is something like "aaa"
        int len=new_s.size();
        vector<int> next(len+1);
        next[0]=-1;
        int k=-1,j=0;
        while(j<len)
        {
            if(k==-1||new_s[k]==new_s[j])
                next[++j]=++k;
            else
                k=next[k];
        }
        int idx=next[len];
        string temp=s.substr(idx);
        reverse(temp.begin(),temp.end());
        return temp+s;
    }
};