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.

解題思路：本題主要是如果所有路徑加起來的gas大於等於cost，那麽一定是存在著解的。所以如果路徑中碰到負值，那下一個gas大於cost的點就是候選點

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

134. Gas Station