加油站

class Solution {
public:
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
        int n1=size(gas);
        for(int k=0;k<n1;k++)
        {
            int i=k,s=gas[k];
            while(i<k+n1)
            {
                if(s<cost[i%n1]) break;
                s+=-cost[i%n1]+gas[(++i)%n1];
            }
            if(i==k+n1) return k;
            k=max(k,i%n1);
        }
        return -1;
    }
};