<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 {
public:
   int p = 31;
   int N = 100000,m = 1e9+7;
   vector<long long int>power;
    int rabin_karp(string a, string b)
    {
        if(a==b)
          return 0;

        int A = a.length();
        int B = b.length();
        vector<long long > h(B+1,0);
        for(int i=0;i<B;i++)
        {
               h[i+1] = (h[i]+(b[i]-'a'+1)*power[i])%m;
        }
        
        long long h_a = 0;
        for(int i=0;i<A;i++)
        {
           h_a = (h_a + (a[i]-'a'+1)*power[i])%m;
        }

        for(int i=0;i+A<=B;i++)
        {
           long long curr = (h[i+A]-h[i]+m)%m;
           if( curr == (h_a*power[i])%m)
           {
              return i;
           }
        }
        return -1;
    }
    int repeatedStringMatch(string a, string b) {
        if(a==b)
          return 1;
        power.resize(N);
        power[0]=1;
        for(int i=1;i<N;i++)
        {
            power[i] = (power[i-1]*p)%m;
        }
        string target="";
        int cnt =0;
        while(target.length() < b.length())
        {
            target = target+a;
            cnt++;
        }

        if(rabin_karp(b,target)!= -1)
        {
           return cnt;
        }
        target = target+a;
        if(rabin_karp(b,target)!= -1){
            return cnt+1;
        }
        return -1;
    }

};
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