Ζητούμενη λύση:

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#include<iostream>
#include<iomanip>
#include "math.h"

using namespace std;

typedef unsigned long long  ull ;

ull perfect(ull);

int main()
{
    
ull    inputnumber, outputnumber ;

do {
    cout<<"\nEnter a positive integer number: "; 
      cin>>inputnumber;
      if (inputnumber>0)
          outputnumber = perfect(inputnumber);
      else
          outputnumber = 0 ;
      if (outputnumber > 0)
          cout<<inputnumber<<" is a perfect square, with square root "<<outputnumber<< endl;
      else
       cout<< inputnumber<<" is NOT a perfect square...\n";
} while (inputnumber>0) ;

return 0;
} 

ull perfect(ull a)
{
    ull    root_of_a, root_of_a_squared;
    root_of_a = truncl(sqrtl(a)) ;          // candidate root should not be altered by truncl
    root_of_a_squared = root_of_a * root_of_a ;
    if (root_of_a_squared == a )        // just found a perfect square!
        return (root_of_a);
    else
        return 0;
}                 

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Βέλτιστη λύση με χρήση μαθηματικών βιβλιοθηκών truncl(), sqrtl() , με διαφορετική υλοποίηση της function perfect()

Σημ: Θα μπορούσαν να χρησιμοποιηθούν και οι βιβλιοθήκες floor(), ceil()

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ull perfect(ull a)
{
    ull    root_of_a, root_of_a_squared;
    root_of_a = truncl(sqrtl(a)) ;       // candidate root should not be altered by truncl
    root_of_a_squared = root_of_a * root_of_a ;
    if (root_of_a_squared == a )      // just found a perfect square!
        return (root_of_a);
    else
        return 0;
}                                                      
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