The author had never even heard of or read Battle Royale before she wrote the books. When someone mentioned the similarities to her, she went to her publisher and asked if she should read/watch Battle Royale. Her publisher told her no, she should not do that until she had finished writing the series.

I've read both - there are definitely some similarities in the themes, but the execution, style, and story are so different! I feel they are both worth reading. I began reading The Hunger Games with skepticism, but after finishing the series, I believe that it is its own thing, there is a great deal to love about it, and it doesn't need to be compared to Battle Royale in order to stand on its own.