介绍下差分约束系统:就是多个2未知数不等式形如(a-b<=k)的形式 问你有没有解,或者求两个未知数的最大差或者最小差 转化为最短路(或最长路) 1:求最小差的时候,不等式转化为b-a>=k的标准形式建图,求最长路 2:求最大差的时候,不等式转化为b-a<=k的标准形式建图,求最短路 然后具体的写的好的博客以供大家参考 1 http://www.cnblogs.com/void/archive/2011/08/26/2153928.html 2 http://blog.csdn.net/…

题目链接:http://poj.org/problem? id=3169 Description Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a straight line waiting for feed. The cows are standin…

描述 Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a straight line waiting for feed. The cows are standing in the same order as they are numbered, and…

Layout Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 6415   Accepted: 3098 Description Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a…

题意:给定一些母牛,要求一个排列,有的母牛距离不能超过w,有的距离不能小于w,问你第一个和第n个最远距离是多少. 析:以前只是听说过个算法,从来没用过,差分约束. 对于第 i 个母牛和第 i+1 个,D[i] - D[i+1] <= 0,  D[j] -D[i ]<= k, D[i] - D[j] <= - k,那么这个题就可以用差分约束来求这个不等式组了. 1.对于差分不等式,a - b <= c ,建一条 b 到 a 的权值为 c 的边,求的是最短路,得到的是最大值(本题求的就…

题目大意:原题链接 当排队等候喂食时,奶牛喜欢和它们的朋友站得靠近些.FJ有N(2<=N<=1000)头奶牛,编号从1到N,沿一条直线站着等候喂食.奶牛排在队伍中的顺序和它们的编号是相同的.因为奶牛相当苗条,所以可能有两头或者更多奶牛站在同一位置上(即间距可能为0).即是说,如果我们想象奶牛是站在一条数轴上的话,允许有两头或更多奶牛拥有相同的横坐标. 一些奶牛相互间存有好感,它们希望两者之间的距离不超过一个给定的数L.另一方面,一些奶牛相互间非常反感,它们希望两者间的距离不小于一个给定的数D.…

题意: 有N头牛, 有以下关系: (1)A牛与B牛相距不能大于k (2)A牛与B牛相距不能小于k (3)第i+1头牛必须在第i头牛前面 给出若干对关系(1),(2) 求出第N头牛与第一头牛的最长可能距离, 若无解输出-1, 若无限长输出-2 分析: 3个关系对应的 <= 式子是: dis[b] - dis[a] <= d(1) dis[a] - dis[b] <= -d(2) dis[i] - dis[i+1] <= -1(2) 目标式:dis[N] - dis[1] <=…

Layout 题目链接: Rhttp://acm.hust.edu.cn/vjudge/contest/122685#problem/S Description Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a straight line waitin…

题目链接:http://poj.org/problem?id=3169 差分约束的解释:http://www.cnblogs.com/void/archive/2011/08/26/2153928.html 我也不是特别理解,要是给你a - b <= k 就建一条b->a权值为k的有向边,要是a - b >= k 就建一条a -> b边权是-k的有向边,要是让你求n到1的最大差,就是让你求1到n的最短距离. 差分约束系统有两种方式可以求解,最短路和最长路.当我们把不等式整理成d[a…

Layout Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 6549   Accepted: 3168 Description Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a…