19. Step1: Enqueue A Step2: Enqueue B Step3:Queue-front Step:1 Enqueue A 1 2 3 4 1 2 3 4 FRONT=0 FRONT REAR REAR=0 A A Enqueue B 1 2 3 4 1 2 3 4 FRONT FRONT REAR REAR B A B Step:2 A FRONT 1 2 3 4 1 2 3 4 FRONT FRONT REAR REAR A A B Step:3 A B
20. Step4: Dequeue Step5: Enqueue C Step6: Queue-Last Dequeue 1 2 3 4 1 2 3 4 FRONT FRONT REAR REAR A B Step:4 A B Enqueue C 1 2 3 4 1 2 3 4 FRONT FRONT REAR REAR C B Step:5 B C LAST 1 2 3 4 1 2 3 4 FRONT FRONT REAR REAR C B Step:6 B C C