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;
    }
};