问题描述：
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.
问题分析：
1.首先应该明确一点：若gas的总量>= cost的总量，那么一定存在一条解决路径。
2.寻找起点：设置起点res = 0；依次迭代到i，若res到i项cost和gas的和小于0，则证明res并不是真正的起点，重设res = i + 1，重复迭代过程。
过程详见代码：
class Solution {public: int canCompleteCircuit(vector<int>& gas, vector<int>& cost) { int size = gas.size();int sum = 0;int res = 0;int total = 0;for (int i = 0; i<size; ++i){sum += gas[i] - cost[i];if (sum<0){total += sum;sum = 0;res = i + 1;}}total += sum;return total<0 ? -1 : res % size; }};