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(int[] gas, int[] cost) {        if (gas == null || cost == null || gas.length == 0 || cost.length == 0) {            return -1;        }        int sum = 0;        int total = 0;        int index = -1;        for(int i = 0; i<gas.length; i++) {            sum += gas[i] - cost[i];            total += gas[i] - cost[i];            if(sum < 0) {                index = i;                sum = 0;            }        }        return total < 0 ? -1 : index + 1;        // index should be updated here for cases ([5], [4]);        // total < 0 is for case [2], [2]    }}