leetcode 214. Shortest Palindrome

题目：

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：

翻转后找相等的最大前后缀，没优化，应该可以不用翻转，直接比较。

class Solution {
public:
    string shortestPalindrome(string s) {
        string t = s;
        reverse(t.begin(),t.end());
        if(t == s) return t;
        int i = t.size()-1;
        for( ;i >=0;i--){
            if(t.substr(t.size()-1-i,i)==s.substr(0,i)) {
                break;
            }
        }
        return t.substr(0,t.size()-1-i) + s;
    }
};

最优：

class Solution {
public:
    string shortestPalindrome(string s) {
        int n = s.length();
        int i = 0, j = n - 1;
        while (j >= 0) {
            if (s[i] == s[j]) ++i;
            
            --j;
        }
        
        if (i == n) return s;
        string suffix = s.substr(i);
        return string(suffix.rbegin(), suffix.rend()) + shortestPalindrome(s.substr(0, i)) + suffix;
    }