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

You have a car with an unlimited gas tank and it costscost[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.

total记录这个旅程的净油量变化，期间可以为负，当走完整个旅程后total是负的，就说明所有的油加起来走不完所有的路，即不可能走完。那么如果total是正的，则一定存在正确的方案。