support test case "3:ABC"
support test case "3:ABCa"
support test case "4:ABCD"

- implementation for the case when one side is occupied
support test case "4:ABCbBD"
support test case "4:ABCDabcA"
support test case "5:NEXUS"
support test case "5:ZYQMyqY"
support test case "5:ABCDbdXYc"