LeetCode-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.

遇到一个无法到达的站则把起点向前推，如果推了一圈则说明无法到达

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int bal=0;
        int start=0;
        for(int i=0;i<gas.size();i++){
            int pos=start+i;
            if(pos>=gas.size())pos-=gas.size();
            bal+=gas[pos];
            bal-=cost[pos];
            while(bal<0){
                start--;
                if(start<0)start=gas.size()-1;
                i++;
                if(i==cost.size())return -1;
                bal+=gas[start];
                bal-=cost[start];
            }
        }
        return start;
    }
};