leetcode:Shortest Palindrome

Shortest Palindrome
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”.
分析
We can construct the following string and run KMP algorithm on it: ST=(s) + (some symbol not present in s) + (reversed string).then we caculate next[j] value of ST.
时间复杂度O(n)，空间复杂度O(n)。

class Solution {
public:
    string shortestPalindrome(string s) {
        string tmp=s;
        int size = s.size();
        reverse(tmp.begin(), tmp.end());
        tmp = s + "#" + tmp;
        vector<int> next(tmp.size()+1, 0);
        KPM_nextJ(tmp, next);
        return tmp.substr(size + 1, size - next[tmp.size()]) + s;
    }
    void KPM_nextJ(const string& str, vector<int>& next){
        int j = 1, k = 0;
        next[0] = -1;
        next[1] = 0;
        int size = str.size();
        while (j < size){
            if (str[j] == str[k]){
                ++k;
                ++j;
                next[j] = k;
            }
            else if (k == 0){
                ++j;
                next[j] = 0;
            }
            else
                k = next[k];
        }
    }
};