已知北京、上海、广州、昆明和西安五城市航线里程如图。分别实现： 1）找到北京到昆明的最短路径。 2）找到北京到昆明，必须途径上海的路径。 3）找到北京到昆明，必须途径上海，但不能去广州的路径。 -known Beijing, Shanghai, Guangzhou, Kunming and Xi'an City route five mileage figure. Were achieved : 1) to Kunming to Beijing to find t