Leetcode 214. Shortest Palindrome

214. Shortest Palindrome

Total Accepted: 21546 

Total Submissions: 105732 

Difficulty: Hard

Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation.

For example:

Given "aacecaaa", return "aaacecaaa".

Given "abcd", return "dcbabcd".

思路：
主要是KMP算法中的前缀字符串和后缀字符串的匹配问题，利用KMP算法的“部分匹配表”。先将原字符串s反转为rev_s，然后产生新的字符串l=rev_s+“#”+s。求出包含l最后一个字符的最长后缀长度，然后求出s的补齐回文字符串。

代码：

class Solution {
public:
    string shortestPalindrome(string s) {
        string rev_s=s;
        reverse(rev_s.begin(),rev_s.end());
        string l=s+"#"+rev_s;
        int i,j;
        vector<int> v(l.size(),0);
        for(i=1;i<l.size();i++){
            j=v[i-1];
            while(j>0&&l[j]!=l[i]) j=v[j-1];
            v[i]=j+(l[j]==l[i]);
        }
        return rev_s.substr(0,s.size()-v[l.size()-1])+s;
    }
};