Q96

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.