leetcode[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.

class Solution {

public:

    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {

        if(gas.empty())return -1;

        int len=gas.size();

        int gasLeft=0;

        int begin=0,end=0;

        int i=0;

        for(int j=0;j<len;j++)

        {

            gasLeft+=gas[i]-cost[i];

            if(j==len-1)break;

            if(gasLeft>=0)

            {

                end++;

                i=end;

            }

            else

            {

                begin=begin==0?len-1:--begin;

                i=begin;

            }

        }

        return gasLeft>=0?begin:-1;

    }

};