Ural 1277 cops ans thieves (minimum cut model), uralcops

Ural 1277 cops ans thieves (minimum cut model), uralcops

Question address: http://acm.timus.ru/problem.aspx? Space = 1 & num = 1277

Here we want to split the points. Split a vertex into I, I '. How can I, j has edges, and creating edges (I, J', inf), (j, I ', inf ). Then each vertex edge (I ', I, R [I]). After edge creation, if we want to block the path from s to f, we must break down some edges. In order to minimize the edge weight, we must destroy the edge with the Edge Weight less than inf, corresponding to the split edge (j '-> j) in the graph ). In fact, this edge represents the vertex right of vertex j. The minimum cut is the answer. There is a special issue when s = f.

VIEW CODE

#include<cstdio>#include<algorithm>#include<iostream>#include<cmath>#include<queue>#include<stack>#include<string>#include<cstring>#include<map>#include<vector>#include<set>#include<ctime>#include<stdlib.h>using namespace std;const int mmax= 210;const int mod=1000000007;const int inf=0x3fffffff;using namespace std;struct node{    int flow;    int en;    int next;}E[40010+mmax];int p[mmax];int num;void init(){    memset(p,-1,sizeof p);    num=0;}void add(int st,int en,int flow){    E[num].en=en;    E[num].flow=flow;    E[num].next=p[st];    p[st]=num++;    E[num].en=st;    E[num].flow=0;    E[num].next=p[en];    p[en]=num++;}int d[mmax];bool vis[mmax];int qq[mmax];int cur[mmax];bool bfs(int st,int en){    memset(vis,0,sizeof vis);    int qcnt=0;    qq[++qcnt]=st;    d[st]=0;    vis[st]=1;    while(qcnt)    {        int x=qq[qcnt];        qcnt--;        for(int i=p[x]; i+1; i=E[i].next)        {            int v=E[i].en;            if(!vis[v]&&E[i].flow)            {                vis[v]=1;                qq[++qcnt]=v;                d[v]=d[x]+1;            }        }    }    return vis[en];}int dfs(int st,int en,int  flow){    if(st==en||flow==0)        return flow;    int f=0,dd;    for(int &i=cur[st]; i+1;i=E[i].next)    {        int v=E[i].en;        if(d[st]+1==d[v]&&(dd=dfs(v,en,min(flow,E[i].flow)))>0)        {            E[i].flow-=dd;            E[i^1].flow+=dd;            flow-=dd;            f+=dd;            if(flow==0)                break;        }    }    return f;}int dinic(int st,int en,int n){    int flow=0;    while(bfs(st,en))    {        for(int i=0;i<=n;i++)            cur[i]=p[i];        flow+=dfs(st,en,inf);    }    return flow;}int main(){    int k,n,m,s,f;    while(cin>>k)    {        init();        scanf("%d %d %d %d",&n,&m,&s,&f);        for(int i=1;i<=n;i++)        {            int x;            scanf("%d",&x);            add(n+i,i,x);        }        for(int i=0;i<m;i++)        {            int u,v;            scanf("%d %d",&u,&v);            add(u,v+n,inf);            add(v,u+n,inf);        }        if(s==f)        {            puts("NO");            continue;        }        int ans=dinic(s,f+n,2*n);        if(ans<=k)            puts("YES");        else            puts("NO");    }    return 0;}