143 lines
3.4 KiB
C++
143 lines
3.4 KiB
C++
#include<iostream>
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#include<cstdio>
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#include<cstring>
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#include<cstdlib>
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#include<cmath>
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#include<algorithm>
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#include<queue>
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#define inf 0x7fffffff
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using namespace std;
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const int maxn=600;
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const int M = 100000+10;
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struct Edge
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{
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int to,cap,next;
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}edge[M*2];
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int head[maxn],edgenum;
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int n,m,from,to,vnum,s,t;
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void add(int u,int v,int cap)
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{
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edge[edgenum].to=v;
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edge[edgenum].cap=cap;
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edge[edgenum].next=head[u];
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head[u]=edgenum++;
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edge[edgenum].to=u;
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edge[edgenum].cap=0;
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edge[edgenum].next=head[v];
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head[v]=edgenum++;
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}
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int level[maxn];
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int gap[maxn];
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void bfs(int to)
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{
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memset(level,-1,sizeof(level));
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memset(gap,0,sizeof(gap));
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level[to]=0;
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gap[level[to] ]++;
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queue<int> Q;
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Q.push(to);
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while (!Q.empty())
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{
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int u=Q.front() ;Q.pop() ;
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for (int i=head[u] ;i!=-1 ;i=edge[i].next)
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{
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int v=edge[i].to;
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if (level[v] != -1) continue;
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level[v]=level[u]+1;
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gap[level[v] ]++;
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Q.push(v);
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}
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}
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}
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int cur[maxn];
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int pre[maxn];
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int SAP(int from,int to)
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{
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bfs(to);
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memset(pre,-1,sizeof(pre));
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memcpy(cur,head,sizeof(head));
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int u=pre[from]=from,flow=0,aug=inf;
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gap[from]=vnum;
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while (level[from]<vnum)
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{
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bool flag=false;
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for (int &i=cur[u] ;i!=-1 ;i=edge[i].next)
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{
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int v=edge[i].to;
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if (edge[i].cap && level[u]==level[v]+1)
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{
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flag=true;
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aug=min(aug,edge[i].cap);
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pre[v]=u;
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u=v;
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if (v==to)
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{
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flow += aug;
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for (u=pre[u] ;v!=from ;v=u ,u=pre[u])
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{
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edge[cur[u] ].cap -= aug;
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edge[cur[u]^1 ].cap += aug;
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}
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aug=inf;
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}
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break;
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}
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}
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if (flag) continue;
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int minlevel=vnum;
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for (int i=head[u] ;i!=-1 ;i=edge[i].next)
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{
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int v=edge[i].to;
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if (edge[i].cap && level[v]<minlevel)
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{
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minlevel=level[v];
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cur[u]=i;
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}
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}
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if (--gap[level[u] ]==0) break;
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level[u]=minlevel+1;
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gap[level[u] ]++;
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u=pre[u];
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}
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return flow;
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}
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int main()
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{
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while (scanf("%d%d",&n,&m)!=EOF)
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{
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if (!n && !m) break;
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memset(head,-1,sizeof(head));
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edgenum=0;
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char c[5],c2[5];
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int u,v;
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int cap,f=M;
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s=0 ;t=n+1 ;from=t+1 ;to=from+1 ;
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vnum=to+1;
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int sum=0;
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for (int i=0 ;i<m ;i++)
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{
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scanf("%s%s%d",&c,&c2,&cap);
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sum += cap;
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if (c[0]=='+') u=s;
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else sscanf(c,"%d",&u);
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if (c2[0]=='-') v=t;
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else sscanf(c2,"%d",&v);
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add(u,v,f-cap);
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add(from,v,cap);
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add(u,to,cap);
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}
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int Maxflow=SAP(from,to);
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int d=edgenum;
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add(t,s,inf);
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Maxflow += SAP(from,to);
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int flag=0;
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for (int i=head[from] ;i!=-1 ;i=edge[i].next)
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{
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if (edge[i].cap) {flag=1;break; }
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}
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if (Maxflow==sum) printf("%d\n",edge[(d^1)].cap);
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else printf("impossible\n");
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}
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return 0;
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}
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