学步园

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.

class Solution {
public:
	int canCompleteCircuit(vector<int> &gas, vector<int> &cost) 
	{
		int sum = 0;
		int total = 0;
		int j = -1;
		for(int i = 0; i < gas.size() ; ++i)
		{
			sum += gas[i]-cost[i];
			total += gas[i]-cost[i];
			if(sum < 0)
			{
				j=i; sum = 0; 
			}
		}
		if(total<0) return -1;
		else return j+1;
	}
};