[题意概述] 修改图中任一一条边的边权,使其加倍,问怎样使修改后图中的1~n的最短路最大.输出最短路的增量. [题解] 先跑一遍dijkstra求出1~n的路径长度,记录下经过的边.枚举这些边进行修改,再跑dijkstra求出某条边修改后的最短路径的长度. #include<cstdio> #include<cstring> #include<algorithm> #define LL long long #define N 200010 #define rg regi…

P2176 [USACO14FEB]路障Roadblock 题目描述 每天早晨,FJ从家中穿过农场走到牛棚.农场由 N 块农田组成,农田通过 M 条双向道路连接,每条路有一定长度.FJ 的房子在 1 号田,牛棚在 N 号田.没有两块田被多条道路连接,以适当的路径顺序总是能在农场任意一对田间行走.当FZ从一块田走到另一块时,总是以总路长最短的道路顺序来走. FJ 的牛呢,总是不安好心,决定干扰他每天早晨的计划.它们在 M 条路的某一条上安放一叠稻草堆,使这条路的长度加倍.牛希望选择一条路干扰使得F…

https://www.luogu.org/problem/show?pid=2176 题目描述 每天早晨,FJ从家中穿过农场走到牛棚.农场由 N 块农田组成,农田通过 M 条双向道路连接,每条路有一定长度.FJ 的房子在 1 号田,牛棚在 N 号田.没有两块田被多条道路连接,以适当的路径顺序总是能在农场任意一对田间行走.当FZ从一块田走到另一块时,总是以总路长最短的道路顺序来走. FJ 的牛呢,总是不安好心,决定干扰他每天早晨的计划.它们在 M 条路的某一条上安放一叠稻草堆,使这条路的长度加倍…

题目描述 每天早晨,FJ从家中穿过农场走到牛棚.农场由 N 块农田组成,农田通过 M 条双向道路连接,每条路有一定长度.FJ 的房子在 1 号田,牛棚在 N 号田.没有两块田被多条道路连接,以适当的路径顺序总是能在农场任意一对田间行走.当FZ从一块田走到另一块时,总是以总路长最短的道路顺序来走. FJ 的牛呢,总是不安好心,决定干扰他每天早晨的计划.它们在 M 条路的某一条上安放一叠稻草堆,使这条路的长度加倍.牛希望选择一条路干扰使得FJ 从家到牛棚的路长增加最多.它们请你设计并告诉它们最大增量…

题目:洛谷P2176. 题目大意:有n个点m条无向边,一个人要从1走到n,他会走最短路.现在可以让一条边的长度翻倍,求翻倍后这个人要多走多少距离. 解题思路:首先可以知道,翻倍肯定是在最短路上的某条边翻,否则他走的路不会变.我们先跑一遍最短路,记录下走的边,再枚举哪条边翻倍,然后跑最短路,记录下答案即可. 此题好像卡SPFA,于是我用堆优化Dijkstra秒杀. 时间复杂度$O(nm\log n)$. C++ Code: #include<cstdio> #include<cstring…

P3102 [USACO14FEB]秘密代码Secret Code 题目描述 Farmer John has secret message that he wants to hide from his cows; the message is a string of length at least 2 containing only the characters A..Z. To encrypt his message, FJ applies a sequence of "operations&…

P3102 [USACO14FEB]秘密代码Secret Code 题目描述 Farmer John has secret message that he wants to hide from his cows; the message is a string of length at least 2 containing only the characters A..Z. To encrypt his message, FJ applies a sequence of "operations&…

P2865 [USACO06NOV]路障Roadblocks 题目描述 Bessie has moved to a small farm and sometimes enjoys returning to visit one of her best friends. She does not want to get to her old home too quickly, because she likes the scenery along the way. She has decided t…

P2237 [USACO14FEB]自动完成Auto-complete 题目描述 Bessie the cow has a new cell phone and enjoys sending text messages, although she keeps making spelling errors since she has trouble typing on such a small screen with her large hooves. Farmer John has agreed…

P3102 [USACO14FEB]秘密代码Secret Code 题目描述 Farmer John has secret message that he wants to hide from his cows; the message is a string of length at least 2 containing only the characters A..Z. To encrypt his message, FJ applies a sequence of "operations&…