<p>You are given a string <code>s</code>. You can convert <code>s</code> to a <span data-keyword="palindrome-string">palindrome</span> by adding characters in front of it.</p>

<p>Return <em>the shortest palindrome you can find by performing this transformation</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> s = "aacecaaa"
<strong>Output:</strong> "aaacecaaa"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = "abcd"
<strong>Output:</strong> "dcbabcd"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= s.length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>s</code> consists of lowercase English letters only.</li>
</ul>

class Solution {
public:
    vector<int> lpstable(string s){
        int n=s.size();
        vector<int> v;
        v.push_back(0);
        int j=0;
        for(int i=1;i<n;i++){
            j=v[i-1];
            while(j>0 && s[j]!=s[i]){
                j=v[j-1];
            }
            if(s[i]==s[j])j++;
            v.push_back(j);
        }
        return v;
    }

    string shortestPalindrome(string s) {
        int n=s.size();
        string rstr=s;
        reverse(rstr.begin(),rstr.end());
        string newstr=s+"#"+rstr;
        vector<int> lps=lpstable(newstr);
        int maxlen=lps.back();
        string extra=s.substr(maxlen,n);
        reverse(extra.begin(),extra.end());
        return extra+s;
    }
};
13900