<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:
    int mulmod(long long b, long long p, long long m){
        long long ans=1;
        b%=m;
        while(p>0){
            if(p%2==1){ans*=b; p--; ans%=m;}
            else{b*=b; b%=m; p/=2;}
        }
        return ans;
    }
    string longestPrefix(string s) {
        int len=0;
        long long e=1000000007;
        int n=s.size();
        int p=113;
        long long invp=mulmod(p,e-2,e);
        long long maskf=0, maskl=0, mc=1;
        for(int i=0; i<n-1; i++){
            maskf+=mc*s[i];
            maskf%=e;
            maskl*=p;
            maskl+=s[n-1-i];
            maskl%=e;
            mc*=p;
            mc%=e;
            if(maskf==maskl)len=i+1;
        }
        return s.substr(0,len);
    }
};
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