UVA 10480 Sabotage

最小割+输出方案

#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<vector>
#include<queue>
#include<algorithm>
using namespace std;

const int maxn=1000+10;
const int INF=0x7FFFFFFF;

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

//求出层次网络
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];
}


//加边
void AddEdge(int from,int to,int cap)
{
    Edge r;
    r.from=from;
    r.to=to;
    r.cap=cap;
    r.flow=0;
    edges.push_back(r);
    Edge d;
    d.from=to;
    d.to=from;
    d.cap=0;
    d.flow=0;
    edges.push_back(d);
    m=edges.size();
    G[from].push_back(m-2);
    G[to].push_back(m-1);
}

//每个阶段来一次DFS增广
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;
}

//多个阶段，多次建立层次网络。
int Maxflow(int ss,int tt)
{
    int flow=0;
    while(BFS())
    {
        memset(cur,0,sizeof(cur));
        flow+=DFS(ss,INF);
    }
    return flow;
}

int N,M,Cost;
int U[maxn],V[maxn];
int FF[maxn];

void dfs(int x)
{
    FF[x]=1;
    for(int i=0; i<CG[x].size(); i++)
        if(!FF[CG[x][i]])
            dfs(CG[x][i]);
}

int main()
{
    while(~scanf("%d%d",&N,&M))
    {
        if(!N&&!M) break;
        edges.clear();
        for(int i=0; i<maxn; i++) G[i].clear();
        for(int i=0; i<maxn; i++) CG[i].clear();
        memset(FF,0,sizeof FF);
        s=1;
        t=2;
        for(int i=1; i<=M; i++)
        {
            scanf("%d%d%d",&U[i],&V[i],&Cost);
            AddEdge(U[i],V[i],Cost);
            AddEdge(V[i],U[i],Cost);
        }
        Maxflow(s,t);
        int Tot=0;
        for(int i=0; i<edges.size(); i=i+2)
        {
            P[Tot].from=edges[i].from;
            P[Tot].to=edges[i].to;
            P[Tot].cap=edges[i].cap;
            P[Tot].flow=edges[i].flow;
            Tot++;
        }
        for(int i=0; i<Tot; i++)
        {
            if(P[i].cap-P[i].flow>0)
                CG[P[i].from].push_back(P[i].to);
            if(P[i].flow>0)
                CG[P[i].to].push_back(P[i].from);
        }
        dfs(s);
        for(int i=1; i<=M; i++)
            if((FF[U[i]]==0&&FF[V[i]]==1)||(FF[V[i]]==0&&FF[U[i]]==1))
                printf("%d %d
",U[i],V[i]);
        printf("
");
    }
    return 0;
}