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) 

    {

        int N=gas.size();

        int sum[N];

        for(int i=0;i<N;i++) sum[i]=gas[i]-cost[i];

        

        //check

        int total=0;

        for(int i=0;i<N;i++) total=total+sum[i];

        if(total<0) return -1;

        

        //find 1st -

        int first=0;

        if(sum[first]>=0)

            while(sum[first]>=0) 

            {

                first=(first+1)%N;

                if(first==0) return 0;

            }

        else

            while(sum[(first-1+N)%N]<0) first=(first-1+N)%N;

            

        int minvalue=0;

        int minindex;

        

        int i=first;

        do

        {

            int total=0;

            while(sum[i]<0)

            {

                total=total+sum[i];

                i=(i+1)%N;

            }

            if(total<minvalue) 

            {

                minvalue=total;

                minindex=i;

            }

            while(sum[i]>=0) i=(i+1)%N;

        }while(i!=first);

        return minindex;

    }

};