Gas Station

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 res=-1;
        int end=gas.size();
        int x=0;
        for (int i=0; i<end; i++) {
            int ii=i%gas.size();
            x+=gas[ii]-cost[ii];
            if (x<0) {
                if (i==gas.size()-1) {
                    return -1;
                }
                if (end<gas.size()*2-1) {
                    end++;
                }
                x=0;
                res=-1;
            }else if (res==-1) {
                res=ii;
            }
        }
        return res;
    }
};