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;
    }
};