Find Duplicate in Array

int Solution::repeatedNumber(const vector<int> &a) {
    // Do not write main() function.
    // Do not read input, instead use the arguments to the function.
    // Do not print the output, instead return values as specified
    // Still have a doubt. Checkout www.interviewbit.com/pages/sample_codes/ for more details
     int sqrtn=sqrt(a.size()-1);
     
     int store[sqrtn];
     
     fill(store,store+sqrtn,0); int check=0;
     
     for(int i=0;i<a.size();i++)
         {
             store[a[i]/sqrtn]++;
             if(store[a[i]/sqrtn]>sqrtn)
                check=a[i]/sqrtn;
             
         }
         
         map<int,int>mp;
         
         for(int i=0;i<a.size();i++)
         {
             if(a[i]>=check && a[i]<=sqrtn+check)
                mp[a[i]]++;
                
            if(mp[a[i]]>1)
              return a[i];
         }
          
     
     
     
       return -1;
     

  
    
    
    
    
    
    
}