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### 题目描述

In a country popular for train travel, you have planned some train travelling one year in advance. The days of the year that you will travel is given as an array `days`. Each day is an integer from `1` to `365`.

Train tickets are sold in 3 different ways:

a 1-day pass is sold for `costs` dollars;
a 7-day pass is sold for `costs` dollars;
a 30-day pass is sold for `costs` dollars.
The passes allow that many days of consecutive travel. For example, if we get a 7-day pass on day 2, then we can travel for 7 days: day 2, 3, 4, 5, 6, 7, and 8.

Return the minimum number of dollars you need to travel every day in the given list of `days`.

### 例子

#### 例子 1

Input: days = [1,4,6,7,8,20], costs = [2,7,15]
Output: 11
Explanation:
For example, here is one way to buy passes that lets you travel your travel plan:
On day 1, you bought a 1-day pass for costs = \$2, which covered day 1.
On day 3, you bought a 7-day pass for costs = \$7, which covered days 3, 4, …, 9.
On day 20, you bought a 1-day pass for costs = \$2, which covered day 20.
In total you spent \$11 and covered all the days of your travel.

#### 例子 2

Input: days = [1,2,3,4,5,6,7,8,9,10,30,31], costs = [2,7,15]
Output: 17
Explanation:
For example, here is one way to buy passes that lets you travel your travel plan:
On day 1, you bought a 30-day pass for costs = \$15 which covered days 1, 2, …, 30.
On day 31, you bought a 1-day pass for costs = \$2 which covered day 31.
In total you spent \$17 and covered all the days of your travel.

### Note

1. `1 <= days.length <= 365`
2. `1 <= days[i] <= 365`
3. `days` is in strictly increasing order.
4. `costs.length == 3`
5. `1 <= costs[i] <= 1000`

### 解题思路

• 一天前的最小花费 + 一天票钱
• 七天前的最小花费 + 七天票钱
• 30天前最小花费 + 30天票钱

• 时间复杂度：O(1)——常数时间（366）
• 空间复杂度：O(1)——常数空间（366）