LeetCode题解–134. Gas Station
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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; }};
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