https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.queue-1?view=netframework-4.7.2
Represents a first-in, first-out collection of objects.
https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.queue-1.peek?view=netframework-4.7.2
Returns the object at the beginning of the Queue<T> without removing it.
适合用来做广度优先遍历Breadth-first search
