VISA question, I am a US citizen traveling for Tourism

In the month of July, I will be traveling to Japan, then to Shanghai / China for three days, then Seoul N Korea for three days, and back to Beijing/China for 6 days and exit China to Singapore.
Do I need a multiple entry VISA or will I get it on arrival at the airport? Please let me know how to proceed or what to do or expect.
Thank you very much