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：

string shortestPalindrome(string s) {

    int end = 0;
    for (int i = 1; i < s.length(); ++i)
    {
        int j = 0, k = i;
        if (end==k+1 && )
        while (s[j] == s[k])
        {
            if (k - j>1)
            {
                ++j;
                --k;
            }
            else
            {
                end = i;
                break;
            }
        }
    }
    string str = s;
    for (int i = end + 1; i < s.length(); ++i)
    {
        str.insert(0, 1, s[i]);
    }
    return str;
}

后来又参考别人的KPM算法的，

string shortestPalindrome(string s) {
    string rev_s = s;
    reverse(rev_s.begin(), rev_s.end());
    string l = s + "#" + rev_s;

    vector<int> p(l.size(), 0);
    for (int i = 1; i < l.size(); i++) {
        int j = p[i - 1];
        while (j > 0 && l[i] != l[j])
            j = p[j - 1];
        p[i] = (j += l[i] == l[j]);
    }

    return rev_s.substr(0, s.size() - p[l.size() - 1]) + s;
}