输入：

1. 拓扑文件,格式如下。注意：

   • 指定：主机名、交换机名、链路名 + 流表项文件名（对应交换机）
   • 无需指定：IP地址、MAC地址（run_exercise.py生成）

   字典格式：key："hosts"，value：是一个list
   {
   	"hosts": [
   		"h1",
   		"h2",
   		"h3"
   	],
   	"switches": {
   		"s1": { "runtime_json" : "s1-runtime.json" },
   		"s2": { "runtime_json" : "s2-runtime.json" },
   		"s3": { "runtime_json" : "s3-runtime.json" }
   	},
   	"links": [
   		["h1", "s1"], ["s1", "s2"], ["s1", "s3"], 
   		["s3", "s2"], ["s2", "h2"], ["s3", "h3"]
   	]
   }