题目大意 给定一个1到n的排列,然后随机选取一个区间,让这个区间内的数随机改变顺序,问这样的一次操作后,该排列的逆序数的期望是多少 首先,一个随机的长度为len的排列的逆序数是(len)*(len-1)/4,这是显然的,因为每种排列倒序一遍就会得到一个新序列,逆序数是len*(len-1)/2 - x(x为原排列的逆序数) 所以我们只需要把所有n*(n-1)/2的区间每种情况都随机化一遍再求逆序对,然后把这个值求和,就可以得到答案了 但是如果用朴素做法,那么复杂度是n^2的 考虑dp[x]表示以…

(一)题意 题目链接:https://www.patest.cn/contests/pat-a-practise/1030 1030. Travel Plan (30) A traveler's map gives the distances between cities along the highways, together with the cost of each highway. Now you are supposed to write a program to help a tra…

Codeforces Round #546 (Div. 2) 题目链接:https://codeforces.com/contest/1136 A. Nastya Is Reading a Book 题意: 一本书有n个章节,之后给出每个章节所在页数范围,然后有一个人来看书看了k页,问还有多少章节没看. 题解: 水题.模拟一下就好了. #include <bits/stdc++.h> using namespace std; typedef long long ll; ; int n; int…

1030 Travel Plan (30 分)   A traveler's map gives the distances between cities along the highways, together with the cost of each highway. Now you are supposed to write a program to help a traveler to decide the shortest path between his/her starting…

Codeforces Round #466 (Div. 2) 题解 A.Points on the line 题目大意: 给你一个数列,定义数列的权值为最大值减去最小值,问最少删除几个数,使得数列的权值小于等于给定值d 题解: 排序,每次挑最大的和最小的,看看最小的能跟多少个数差\(>d\),看看最大的能跟多少个数差\(>d\),取个数大的那个删除.复杂度\(n^2 + n\log n\) 代码 #include <iostream> #include <cstdio>…

Codeforces Round #677 (Div. 3) 题解 A. Boring Apartments 题目 题解 简单签到题,直接数,小于这个数的\(+10\). 代码 #include <bits/stdc++.h> using namespace std; #define ll long long #define IOS ios_base::sync_with_stdio(0); cin.tie(0);cout.tie(0); const int maxn = 2e5 + 10;…

Codeforces Round #182 (Div. 1)题解 A题:Yaroslav and Sequence1 题意: 给你\(2*n+1\)个元素,你每次可以进行无数种操作,每次操作必须选择其中n个元素改变符号,你的目的是使得最后所有数的和尽量大,问你答案是多少 题解: 感觉上就是构造题,手动玩一玩就知道,当n为奇数的时候,你可以通过三次操作,使得只会改变一个负数的符号.同理n为偶数的时候,每次要改变两个负数的符号. 所以答案如下: 当n为奇数的时候,答案为所有数的绝对值和 当n为偶数的…

1030 Travel Plan (30)(30 分) A traveler's map gives the distances between cities along the highways, together with the cost of each highway. Now you are supposed to write a program to help a traveler to decide the shortest path between his/her startin…

1030 Travel Plan (30 分) A traveler's map gives the distances between cities along the highways, together with the cost of each highway. Now you are supposed to write a program to help a traveler to decide the shortest path between his/her starting ci…

1030. Travel Plan (30) A traveler's map gives the distances between cities along the highways, together with the cost of each highway. Now you are supposed to write a program to help a traveler to decide the shortest path between his/her starting cit…