There areNgas stations along a circular route, where the amount of gas at stationiisgas[i].

You have a car with an unlimited gas tank and it costscost[i]of gas to travel from stationito 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.

tag: two pointer

class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int n = gas.length;

        int start = n - 1, end = 0;
        int sum = gas[start] - cost[start];
        while (start > end){
            if (sum >= 0){
                sum += gas[end] - cost[end];
                end++;
            }
            else{
                start--;
                sum += gas[start] - cost[start];
            }
        }

        return sum >= 0 ? start : -1;
    }
}