Gas Station

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.
解析：
从起点开始当油量小于0时，下个点为起点

答案：

class Solution {
public:
int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
vector<int>v(gas.size(),0);
int sum=0;
for (int i=0; i<gas.size(); i++)
{
v[i]=gas[i]-cost[i];
sum+=v[i];
}

if (sum<0) return -1;
int ans=0;
sum=0;
for (int i=0; i<v.size(); i++)
{
if ((sum)>0)
{
sum+=v[i];
}
else{
ans=i;
sum=v[i];
}
}

return ans;

}
};