<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:
    long long lps(string &s) {
    vector<long long> lps(s.size(), 0);
    int i = 0;
    int j = 1;
    int n = s.size();
    
    while (j < n) {
        if (s[i] == s[j]) {
            lps[j] = i + 1;
            i++;
            j++;
        } else {
            if (i == 0) {
                lps[j] = 0;
                j++;
            } else {
                i = lps[i - 1];
            }
        }
    }
    
    return lps[n-1];
}

string shortestPalindrome(string s) {
    string head = s + "#" + string(s.rbegin(), s.rend());
    long long ans = lps(head);
    string add = s.substr(ans);
    reverse(add.begin(), add.end());
    return add + s;
}

};
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