leetcode：Gas Station 【Java】

一、问题描述

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.

二、问题分析

设置两个变量：

totalGasRemain:所有gas[i] - cost[i]的和，用于验证这个数组是否有解，若大于零则说明可以绕一圈，否则不能绕一圈。

gapRemain:从gas站i出发到gas站j还剩多少gas，若大于零则说明到从i可以到j，此时继续往下走，有可能从i出发可以绕一圈。若小于零，则说明从i出发不能绕一圈，此时重新探测从j是否可以绕一圈。

之所以上面的方法可行是因为题目中已经明确了该题有唯一的解。

三、算法代码

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