【To Understand! 回文串6 KMP算法】LeetCode 214. Shortest Palindrome

LeetCode 214. Shortest Palindrome

Solution1：最笨的方法。时间复杂度$O(n^2)$
竟然能AC，xixixi

class Solution {
public:
    string shortestPalindrome(string s) { //最短的回文串，则对称轴要尽可能往后！
        if (s.size() <= 1) return s;
        int i = s.size() - 1;
        for (; i >= 0; i--) {
            if (isPalin(s, 0, i)) break;
        }
        if (i == s.size() - 1) return s;
        else {
            string s_copy = s;
            string pre = s_copy.substr(i + 1);
            reverse(pre.begin(), pre.end());
            return pre + s;
        }
    }

    bool isPalin(string &str, int begin, int end) {
        while (begin < end) {
            if (str[begin] != str[end])
                return false;
            else {
                begin++;end--;
            }
        }
        return true;
    }
};

Solution2：
参考网址：http://www.cnblogs.com/grandyang/p/4523624.html
利用KMP算法
参考网址：
[1]http://www.cnblogs.com/grandyang/p/4523624.html
[2]https://blog.youkuaiyun.com/v_july_v/article/details/7041827

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