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.

有两点思路：

1、如果A不能到达B，那么AB之间的点也到不了B。(这里B是A第一个不能到达的点)

2、如果油量的和大于消耗的和，肯定有一个解

代码如下：

public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int total = 0, tank = 0, index = 0;
        for (int i = 0; i < gas.length; i++) {
            tank = tank - cost[i] + gas[i];
            if (tank < 0) {
                index = i + 1;
                total += tank;
                tank = 0;
            }
        }
        return total + tank >= 0? index: -1;
    }
}