* Clone Graph 
Clone an undirected graph. Each node in the graph contains a label and
a list of its neighbors.
* Gas Station

There are N gas stations along a circular route, where the amount of gas
at station i is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to
travel from station i to its next station (i+1). You begin the journey
with an empty tank at one of the gas stations.

Return the starting gas station's index if you can travel around the
circuit once, otherwise return -1.
Merge branch 'master' of https://github.com/wh688/leetcode.git