214. 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".

刚开始写的代码运行超时。

如下：

 1 string shortestPalindrome(string s) {
 2 
 3     int end = 0;
 4     for (int i = 1; i < s.length(); ++i)
 5     {
 6         int j = 0, k = i;
 7         if (end==k+1 && )
 8         while (s[j] == s[k])
 9         {
10             if (k - j>1)
11             {
12                 ++j;
13                 --k;
14             }
15             else
16             {
17                 end = i;
18                 break;
19             }
20         }
21     }
22     string str = s;
23     for (int i = end + 1; i < s.length(); ++i)
24     {
25         str.insert(0, 1, s[i]);
26     }
27     return str;
28 }

后来又参考别人的KPM算法的，

 1 string shortestPalindrome(string s) {
 2     string rev_s = s;
 3     reverse(rev_s.begin(), rev_s.end());
 4     string l = s + "#" + rev_s;
 5 
 6     vector<int> p(l.size(), 0);
 7     for (int i = 1; i < l.size(); i++) {
 8         int j = p[i - 1];
 9         while (j > 0 && l[i] != l[j])
10             j = p[j - 1];
11         p[i] = (j += l[i] == l[j]);
12     }
13 
14     return rev_s.substr(0, s.size() - p[l.size() - 1]) + s;
15 }