Day 11 C. Make It Good

最新推荐文章于 2021-08-06 10:35:00 发布

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最新推荐文章于 2021-08-06 10:35:00 发布

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分类专栏： Codeforces 文章标签：贪心算法

本文链接：https://blog.youkuaiyun.com/Jackey_tao/article/details/119059300

给定一个整数数组，目标是找到需要删除的最短前缀，使得剩余数组可以经过一定次数的从两端取数操作变为非递减数组。题目提供了示例和输入输出格式，并给出了一些测试用例。

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Problem:
You are given an array a consisting of n integers. You have to find the length of the smallest (shortest) prefix of elements you need to erase from a to make it a good array. Recall that the prefix of the array a=[a1,a2,…,an] is a subarray consisting several first elements: the prefix of the array a of length k is the array [a1,a2,…,ak] (0≤k≤n).

The array b of length m is called good, if you can obtain a non-decreasing array c (c1≤c2≤⋯≤cm) from it, repeating the following operation m times (initially, c is empty):

select either the first or the last element of b, remove it from b, and append it to the end of the array c.
For example, if we do 4 operations: take b1, then bm, then bm−1 and at last b2, then b becomes [b3,b4,…,bm−3] and c=[b1,bm,bm−1,b2].

Consider the following example: b=[1,2,3,4,4,2,1]. This array is good because we can obtain non-decreasing array c from it by the following sequence of operations:

take the first element of b, so b=[2,3,4,4,2,1], c=[1];
take the last element of b, so b=[2,3,4,4,2], c=[1,1];
take the last element of b, so b=[2,3,4,4], c=[1,1,2];
take the first element of b, so b=[3,4,4], c=[1,1,2,2];
take the first element of b, so b=[4,4], c=[1,1,2,2,3];
take the last element of b, so b=[4], c=[1,1,2,2,3,4];
take the only element of b, so b=[], c=[1,1,2,2,3,4,4] — c is non-decreasing.
Note that the array consisting of one element is good.

Print the length of the shortest prefix of a to delete (erase