笔试-判断是否素数

    public static final boolean isPrimaryNum(int num) 
    { 
        if(num <= 1) 
        { 
            return false; 
        } 

        if(num == 2 || num == 3) 
        { 
            return true; 
        } 

        int sqrt = (int)Math.sqrt(num); 

        if(num % 2 == 0) 
            return false; 

        for(int i = 3; i <= sqrt; i += 2) 
        { 
            if(num % i == 0) 
                return false; 
        } 
        return true; 
    }