[POJ 1149] PIGS【网络流】
Problem:
| Time Limit: 1000MS | Memory Limit: 10000K |
Description
Mirko works on a pig farm that consists of M locked pig-houses and Mirko can't unlock any pighouse because he doesn't have the keys. Customers come to the farm one after another. Each of them has keys to some pig-houses and wants to buy a certain number of pigs.
All data concerning customers planning to visit the farm on that particular day are available to Mirko early in the morning so that he can make a sales-plan in order to maximize the number of pigs sold.
More precisely, the procedure is as following: the customer arrives, opens all pig-houses to which he has the key, Mirko sells a certain number of pigs from all the unlocked pig-houses to him, and, if Mirko wants, he can redistribute the remaining pigs across the unlocked pig-houses.
An unlimited number of pigs can be placed in every pig-house.
Write a program that will find the maximum number of pigs that he can sell on that day.
All data concerning customers planning to visit the farm on that particular day are available to Mirko early in the morning so that he can make a sales-plan in order to maximize the number of pigs sold.
More precisely, the procedure is as following: the customer arrives, opens all pig-houses to which he has the key, Mirko sells a certain number of pigs from all the unlocked pig-houses to him, and, if Mirko wants, he can redistribute the remaining pigs across the unlocked pig-houses.
An unlimited number of pigs can be placed in every pig-house.
Write a program that will find the maximum number of pigs that he can sell on that day.
Input
The first line of input contains two integers M and N, 1 <= M <= 1000, 1 <= N <= 100, number of pighouses and number of customers. Pig houses are numbered from 1 to M and customers are numbered from 1 to N.
The next line contains M integeres, for each pig-house initial number of pigs. The number of pigs in each pig-house is greater or equal to 0 and less or equal to 1000.
The next N lines contains records about the customers in the following form ( record about the i-th customer is written in the (i+2)-th line):
A K1 K2 ... KA B It means that this customer has key to the pig-houses marked with the numbers K1, K2, ..., KA (sorted nondecreasingly ) and that he wants to buy B pigs. Numbers A and B can be equal to 0.
The next line contains M integeres, for each pig-house initial number of pigs. The number of pigs in each pig-house is greater or equal to 0 and less or equal to 1000.
The next N lines contains records about the customers in the following form ( record about the i-th customer is written in the (i+2)-th line):
A K1 K2 ... KA B It means that this customer has key to the pig-houses marked with the numbers K1, K2, ..., KA (sorted nondecreasingly ) and that he wants to buy B pigs. Numbers A and B can be equal to 0.
Output
The first and only line of the output should contain the number of sold pigs.
Sample Input
3 3 3 1 10 2 1 2 2 2 1 3 3 1 2 6
Sample Output
7
Source
Croatia OI 2002 Final Exam - First day
Solution:
这道题的难点在建图。
我们可以发现，对于每个顾客，我们要限制其购买数量，可以从该顾客到汇点连一条容量为购买数量限制的边。
而每个猪圈的第一个顾客可以买该猪圈原有的猪，所以我们可以从源点到每个猪圈的第一个顾客连一条容量为初始猪的数量的边。
同时，由于允许调动，后到的顾客若和先到的顾客有同一个猪圈的钥匙，就可以购买先到的顾客所剩下的所有猪，所以我们可以类比链表的思想，从拥有同一猪圈钥匙的前一个顾客到后一个顾客连一条容量为 ∞ 的边。
建图完毕后跑 Dinic 算出最大流即为答案。
Code: O(N2E), E为总边数 [708K, 16MS]
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;
int M, N;
int pig[1005];
int prev[1005];
struct Edge{
int np, flow, cap;
Edge *nxt, *rev;
};
struct Graph{
Edge *V[105], E[10005];
int tope;
inline void addedge(int u, int v, int cap){
E[++tope].np = v, E[tope].flow = 0, E[tope].cap = cap;
E[tope].nxt = V[u], E[tope].rev = &E[tope + 1], V[u] = &E[tope];
E[++tope].np = u, E[tope].flow = 0, E[tope].cap = 0;
E[tope].nxt = V[v], E[tope].rev = &E[tope - 1], V[v] = &E[tope];
}
} G;
int lev[102];
Edge *arc[102];
int q[102], fr = 0, re = 0;
inline bool Dinic_BFS(){
memset(lev, -1, sizeof(lev));
fr = re = 0, q[re++] = 0, lev[0] = 0;
while(fr != re){
int u = q[fr++];
for(register Edge *ne = G.V[u]; ne; ne = ne->nxt)
if(lev[ne->np] == -1 && ne->cap > ne->flow)
lev[ne->np] = lev[u] + 1, q[re++] = ne->np;
}
return lev[101] > -1;
}
inline int Dinic_DFS(int u, int curflow){
if(u == 101) return curflow;
int remflow = curflow;
for(register Edge *ne = arc[u]; ne; ne = ne->nxt){
arc[u] = ne;
if(lev[ne->np] == lev[u] + 1 && ne->cap > ne->flow){
int dflow = Dinic_DFS(ne->np, min(remflow, ne->cap - ne->flow));
if(dflow){
ne->flow += dflow, ne->rev->flow -= dflow;
remflow -= dflow;
}
}
}
return curflow - remflow;
}
inline int Dinic(){
int maxflow = 0;
while(Dinic_BFS()){
for(register int i = 0; i <= 101; i++) arc[i] = G.V[i];
maxflow += Dinic_DFS(0, 0x3f3f3f3f);
}
return maxflow;
}
int main(){
scanf("%d%d", &M, &N);
for(register int i = 1; i <= M; i++) scanf("%d", pig + i);
for(register int i = 1; i <= N; i++){
int A, B, K;
scanf("%d", &A);
while(A--){
scanf("%d", &K);
if(!prev[K]) G.addedge(0, i, pig[K]);
else G.addedge(prev[K], i, 0x3f3f3f3f);
prev[K] = i;
}
scanf("%d", &B);
G.addedge(i, 101, B);
}
int maxflow = Dinic();
printf("%d\n", maxflow);
return 0;
}
发表评论