134. 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.

解题思路：本题主要是如果所有路径加起来的gas大于等于cost，那么一定是存在着解的。所以如果路径中碰到负值，那下一个gas大于cost的点就是候选点

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;
    }
};