gas stations

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.

public int canCompleteCircuit(int[] gas, int[] cost) {
        if (gas == null) {
            return -1;
        }
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int count = gas.length;
        
        int n = 0;
        int gasInCar = 0;
        int begin = 0;
        int end = 0;
        int i = 0;
        while (n < count - 1) {
            gasInCar += gas[i] - cost[i];
            if (gasInCar >=0) {//forward
                end++;
                i=end;
            } else {
                begin--;
                if (begin < 0) {
                    begin = count - 1;
                }
                i = begin;
            }
            
            n++;
        }
        
        gasInCar += gas[i] - cost[i];
        
        if (gasInCar >= 0) {
            return begin;
        } else {
            return -1;
        }
        
    }