<p>A string is called a <strong>happy prefix</strong> if is a <strong>non-empty</strong> prefix which is also a suffix (excluding itself).</p>

<p>Given a string <code>s</code>, return <em>the <strong>longest happy prefix</strong> of</em> <code>s</code>. Return an empty string <code>&quot;&quot;</code> if no such prefix exists.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;level&quot;
<strong>Output:</strong> &quot;l&quot;
<strong>Explanation:</strong> s contains 4 prefix excluding itself (&quot;l&quot;, &quot;le&quot;, &quot;lev&quot;, &quot;leve&quot;), and suffix (&quot;l&quot;, &quot;el&quot;, &quot;vel&quot;, &quot;evel&quot;). The largest prefix which is also suffix is given by &quot;l&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;ababab&quot;
<strong>Output:</strong> &quot;abab&quot;
<strong>Explanation:</strong> &quot;abab&quot; is the largest prefix which is also suffix. They can overlap in the original string.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s</code> contains only lowercase English letters.</li>
</ul>

class Solution {
public:
    string longestPrefix(string s) {
        int n = s.length();
        vector<int> hash(n,0);
        vector<long int> pow(n,0);

        int p = 13;
        long int curHash = 0;
        int mod = 1e9 + 7;
        pow[0] = 1;

        for(int i=0;i<n;i++)
        {
            curHash = ((curHash*1ll*p)%mod + (s[i]-'a'))%mod;
            hash[i] = curHash;
            if(i+1<n)
                pow[i+1] = (pow[i]*1ll*p)%mod;
        }

        for(int i=0;i<n-1;i++)
        {
            curHash = (curHash + mod - ((s[i]-'a')*1ll*pow[n-i-1])%mod)%mod;
            if(curHash == hash[n-i-2])
            {
                if(s.substr(0,n-i-1) == s.substr(i+1,n-i-1))
                {
                    return s.substr(0,n-i-1);
                }
            }
        }
        return "";
    }
};
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