POJ 3498 March of the Penguins(枚举+最大流)
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题意:在X,Y坐标系中有N(N<=100)个冰块...有些冰块上有若干只企鹅..每只企鹅一次最多跳M距离..一个冰块在有Mi个企鹅离开..就会消失..问有哪些冰块可以作为集合点..就是所有企鹅都能成功到这个冰块上来.
思路:枚举每一块冰块,看看最大流能否等于企鹅总数即可
建图:把每块冰分成两个点i和i+n. i表示进入i冰块的点(可以有无数企鹅过来,所以从别的冰到i有边,容量为INF) i+n表示从i冰块出去的点(最多只能有Mi企鹅从这跳出去,所以从i到i+n有边,且容量为Mi)
从源点S到i有边(S, i, i点初始企鹅数).
从i到i+n有边(i, i+n, Mi). 表示第i块冰最多只有Mi个企鹅能跳走.
因为i+n表示的是第i个跳走的点,所以如果冰块i和j之间的距离<=企鹅能跳跃的距离M,有边(i+n, j, INF)
假设我们当前枚举第x块冰块作为集合点,那么(x分成x和x+n两个点)x点就是汇点(不是x+n点哦),我们只要计算到x点的流量是否==企鹅总数即可.
Trick:如果有拆点的话在初始化的时候记得修改初始化函数..这个傻逼错误我调了一个多小时...#include <cstdio>#include <queue>#include <cstring>#include <iostream>#include <cstdlib>#include <algorithm>#include <vector>#include <map>#include <string>#include <set>#include <ctime>#include <cmath>#include <cctype>using namespace std;#define maxn 550#define INF 1e9#define LLINF 1LL<<60#define LL long longint cas=1,T;struct Edge{int from,to,cap,flow;Edge(int u,int v,int c,int f):from(u),to(v),cap(c),flow(f){}};int n,m;struct Dinic{//int n,m; int s,t;vector<Edge>edges; //边数的两倍vector<int> G[maxn]; //邻接表,G[i][j]表示结点i的第j条边在e数组中的序号bool vis[maxn]; //BFS使用int d[maxn]; //从起点到i的距离int cur[maxn]; //当前弧下标void init(){ for (int i=0;i<=2*n+2;i++) //初始化!!!如果有拆点记得这里要改 G[i].clear(); edges.clear(); // memset(d,0,sizeof(d));}void AddEdge(int from,int to,int cap){edges.push_back(Edge(from,to,cap,0));edges.push_back(Edge(to,from,0,0)); //反向弧int mm=edges.size();G[from].push_back(mm-2);G[to].push_back(mm-1);}bool BFS(){memset(vis,0,sizeof(vis));queue<int>q;q.push(s);d[s]=0;vis[s]=1;while (!q.empty()){int x = q.front();q.pop();for (int i = 0;i<G[x].size();i++){Edge &e = edges[G[x][i]];if (!vis[e.to] && e.cap > e.flow){vis[e.to]=1;d[e.to] = d[x]+1;q.push(e.to);}}}return vis[t];}int DFS(int x,int a){if (x==t || a==0)return a;int flow = 0,f;for(int &i=cur[x];i<G[x].size();i++){Edge &e = edges[G[x][i]];if (d[x]+1 == d[e.to] && (f=DFS(e.to,min(a,e.cap-e.flow)))>0){e.flow+=f;edges[G[x][i]^1].flow-=f;flow+=f;a-=f;if (a==0)break;}}return flow;}int Maxflow(int s,int t){this->s=s;this->t=t;int flow = 0;while (BFS()){memset(cur,0,sizeof(cur));flow+=DFS(s,INF);}return flow;}}dc;int sum;double limit;struct Node{double x,y;int now; //有多少只企鹅在冰上int num; //一块冰可以让多少只企鹅跳}pie[maxn];double getdist(Node a,Node b){return ((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}bool solve(int ice){dc.init(); for (int i = 1;i<=n;i++){if (pie[i].now) dc.AddEdge(0,i,pie[i].now);}for (int i = 1;i<=n;i++){dc.AddEdge(i,i+n,pie[i].num);}for (int i = 1;i<=n;i++)for (int j = i+1;j<=n;j++)if (getdist(pie[i],pie[j]) <= limit*limit){ dc.AddEdge(i+n,j,INF); dc.AddEdge(j+n,i,INF);}return dc.Maxflow(0,ice) == sum;}int main(){//freopen("in","r",stdin);scanf("%d",&T);while (T--){scanf("%d%lf",&n,&limit);sum=0;for (int i = 1;i<=n;i++){scanf("%lf%lf%d%d",&pie[i].x,&pie[i].y,&pie[i].now,&pie[i].num);sum+=pie[i].now; //统计企鹅的数量}vector <int> ans;ans.clear(); for (int i = 1;i<=n;i++){if (solve(i))ans.push_back(i);}if (ans.size()==0){printf("-1\n");}else {for (int i = 0;i<ans.size()-1;i++) printf("%d ",ans[i]-1);printf("%d\n",ans[ans.size()-1]-1);}}//printf("time=%.3lf",(double)clock()/CLOCKS_PER_SEC);return 0;}
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