LeetCode题解–134. Gas Station

链接

LeetCode题目：https://leetcode.com/problems/gas-station/

难度：Medium

题目

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.

题目大意是有N个环状排列的加油站，给定每个加油站的汽油量和开到下个加油站的花费量，判断能否环状遍历一次并返回起点位置。

分析

一开始想的是直接模拟，将每个加油站做为起点尝试能否走完全程。时间复杂度是O(n^2)，在最后一个测试用例超时。
O(n)的做法是设置两个变量，sum判断当前位置能否到达，total代表总汽油量和总花费量的差，显然只有total<0的情况下不能走完全程，而起点在最后一个令sum<0的位置的下一个。

代码

class Solution {
public:
    int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
        int len = (int) gas.size();

        for (int i = 0; i < len; i++) gas[i] -= cost[i];

        int total = 0, sum = 0, res = -1;
        for (int i = 0; i < len; i++) {
            total += gas[i];
            sum += gas[i];
            if (sum < 0) {
                sum = 0;
                res = i + 1;
            }
        }
        return total < 0 ? -1 : res;


    }
};