<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:
    void generate_h( vector<int> &h , string s) { 
        h.push_back(-1) ;
        int i = 1 , j = 0 ;
        while ( i < s.size()) { 
            if ( s[i] != s[j]) { 
                h.push_back(j) ;
                j = h[j] ;
                while ( j >= 0 && s[i] != s[j] ) { 
                    j = h[j] ;
                }
            }
            else { 
                h.push_back(h[j]) ;
            }
            i++ ; j++ ;
        }
        h.push_back(j) ;
    }


    string shortestPalindrome(string s) {  

    string s3 = s ;
    reverse(s3.begin() , s3.end()) ;
    string s1 = s +  "#" + s3 ;

    vector<int> h ;
    generate_h( h , s1) ;
    int k = h[ s1.size() ] ; 
    string s2 = s.substr(k) ;
    reverse(s2.begin() , s2.end()) ;
    return s2 + s ;
    }



};
14000