poj 1637 Sightseeing tour 混和欧拉图 dinic

#pragma comment(linker, "/STACK:102400000,102400000")
#include<stdio.h>
#include<string.h>
#include<queue>
#include<vector>
using namespace std;
const int N=1024;
const int inf=1<<24;

struct Edge
{
    int from,to,cap,flow;
};
vector<Edge>edges;
vector<int>G[N];
int n,m,s,t;
int vis[N];
int d[N];
int cur[N];

void AddEdge(int from,int to,int cap)
{
    Edge tp;
    tp.from=from,tp.to=to,tp.cap=cap,tp.flow=0;
    edges.push_back(tp);

    tp.from=to,tp.to=from,tp.cap=0,tp.flow=0;
    edges.push_back(tp);

    int g_size=edges.size();
    G[from].push_back(g_size-2);
    G[to].push_back(g_size-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 st,int ed)
{
    int flow=0;
    while(BFS())
    {
        memset(cur,0,sizeof(cur));
        flow+=DFS(st,inf);
    }
    return flow;
}

int main()
{

    int i,j,_,u,v,c,flow,f,mp[N][N],in[N],out[N];
    scanf("%d",&_);
    while(_--)
    {
        scanf("%d%d",&n,&s);
        memset(mp,0,sizeof(mp));
        memset(in,0,sizeof(in));
        memset(out,0,sizeof(out));
        edges.clear();
        for(i=0;i<N;i++) G[i].clear();
        for(i=0;i<s;i++)
        {
            scanf("%d%d%d",&u,&v,&c);
            if(c==0) mp[u][v]++;
            out[u]++;
            in[v]++;
        }
        f=1;
        s=0;
        t=n+1;
        for(i=1,flow=0;i<=n;i++)
        {
            int tp=in[i]-out[i];
            if(tp>0) flow+=tp/2;
            else tp=-tp;
            if(tp%2==1) {f=0;break;}

        }
        if(f==0) {printf("impossible\n");continue;}
        for(i=1;i<=n;i++)
        {
            if(in[i]>out[i])
                AddEdge(s,i,(in[i]-out[i])/2);
            else
                AddEdge(i,t,(out[i]-in[i])/2);
            for(j=1;j<=n;j++)
                if(mp[i][j])
                    AddEdge(j,i,mp[i][j]);
        }
        if(Maxflow(s,t)==flow)
            printf("possible\n");
        else
            printf("impossible\n");
    }
    return 0;
}

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