134. 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.

Note:

The solution is guaranteed to be unique.

Thoughts:

1. If sum of gas is more than sum of cost, then there must be a solution. And the question guaranteed that the solution is unique. So the first possible start point is the right one. 
2. The tank should never be negative, so restart whenever there is a negative number.

Java Code: