说明：  中兴面试试题 曹操南下攻打刘备，刘备派关羽守锦州，关羽派张飞去守城门。刘备又派诸葛亮去向孙权求援。孙权派兵攻打曹操！ 请画出UML图
(ZTE interview questions southward to attack Liu Bei Cao Cao, Liu Bei and Guan Yu Shou send jinzhou, Zhang Fei and Guan Yu sent to Shoucheng door. Liu Bei sent Zhuge Liang for help where Sun Quan. Sun Quan sent troops to attack Cao Cao! Please draw UML diagram)