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.

比较直观的是从每个地点开始，但是这题可以只遍历一遍，假设从1开始到4，发现不能走了，说明2，3，4开始也不行。最后检查下，总油量是否大于总消耗。

public class Solution {
    //Time: O(n)  Space: O(1);
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int sum = 0;
        int cur = 0;
        int index = 0;
        for (int i = 0; i < gas.length; i++) {
            cur += gas[i] - cost[i];
            sum += gas[i] - cost[i];
            if (cur < 0) {
                cur = 0;
                index = i + 1;
            }
        }
        return sum < 0 ? -1 : index;
    }
}