Both are fine. 'Of' is optional, especially in less formal situations.
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I came across this question when I read the Lord of the Flies. I then noticed that the author used both phrases. As I found out it does not make much difference. By the way, I highly recommend reading this book. I was very impressed with this story. It really shows the whole essence of society. And this book has revealed many problems of modern humanity. Many thanks to the author for this.