Gas Station

Question:

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.

Solution:

class Solution {
public:
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
    int sum=0;
    int index;
    bool flag=false;
    int count=0;
    for(int i=0;count<=gas.size();i++)
    {
        if(flag==false)
            index=i;
        flag=true;
        if(i<gas.size())
            sum+=gas[i]-cost[i];
        else
            sum+=gas[i-gas.size()]-cost[i-gas.size()];
        count++;
        if(sum<0)
        {
            flag=false;
            sum=0;
            count=0;
        }
        if(count==gas.size()) return index;
        if(i>=2*gas.size()-1) {return -1;}
    }
    }
};