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 the shortest path. 2) find Beijing to Kunming, Shanghai must channel's path. 3) to find Beijing to Kunming, Shanghai must channels, but can not go to Guangzhou's path.