<p>Given two strings <code>a</code> and <code>b</code>, return <em>the minimum number of times you should repeat string </em><code>a</code><em> so that string</em> <code>b</code> <em>is a substring of it</em>. If it is impossible for <code>b</code>​​​​​​ to be a substring of <code>a</code> after repeating it, return <code>-1</code>.</p>

<p><strong>Notice:</strong> string <code>&quot;abc&quot;</code> repeated 0 times is <code>&quot;&quot;</code>, repeated 1 time is <code>&quot;abc&quot;</code> and repeated 2 times is <code>&quot;abcabc&quot;</code>.</p>

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

<pre>
<strong>Input:</strong> a = &quot;abcd&quot;, b = &quot;cdabcdab&quot;
<strong>Output:</strong> 3
<strong>Explanation:</strong> We return 3 because by repeating a three times &quot;ab<strong>cdabcdab</strong>cd&quot;, b is a substring of it.
</pre>

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

<pre>
<strong>Input:</strong> a = &quot;a&quot;, b = &quot;aa&quot;
<strong>Output:</strong> 2
</pre>

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

<ul>
	<li><code>1 &lt;= a.length, b.length &lt;= 10<sup>4</sup></code></li>
	<li><code>a</code> and <code>b</code> consist of lowercase English letters.</li>
</ul>

class Solution {
    long long mod = 1e9+7;
    bool rabin_karp(string &s , string &t){
        long long n = s.size();
        long long m = t.size();
        if(n<m) return 0;
        vector<long long> p(n+1,1);
        for(long long i=1;i<n+1;i++){
            p[i] = (p[i-1]*31)%mod;
        }
        vector<long long> hsh(n+1,0);
        for(long long i=0;i<n;i++){
            hsh[i+1] = (hsh[i]+(p[i]*(s[i]-'a'+1)%mod))%mod;
        }
        long long hshtotal=0;
        for(long long i=0;i<m;i++){
            hshtotal = (hshtotal%mod + (t[i]-'a'+1)*p[i]%mod)%mod;
        }
        for(long long i=0;i<=(n-m);i++){
            long long x = (hsh[i+m]-hsh[i]+mod)%mod;
            if(x==(hshtotal*p[i])%mod) return 1;
        }

        return 0;
    }
public:
    int repeatedStringMatch(string a, string b) {
        long long n = a.size();
        long long m = b.size();
        string temp="";
        for(long long i=0;i<((m/n)+2);i++){
            temp+=a;
            if(rabin_karp(temp,b)){
                return i+1;
            }
        }

        return -1;
    }
};
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