zoj 2587 Unique Attack 网络流

 看最小割是否唯一。就是在残余网络上扫一遍看是否所有边都能被访问到

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
#define inf 10000000
const int maxn=1500;
int n,m;
int from[maxn],to[maxn];
int level[maxn],que[maxn];
int head[maxn],lon;
int min(int a,int b)
{
	if(a<b) return a;
	else return b;
}
struct edge
{
    int next,from,to,c;
}e[45000];
void edgeini()
{
    memset(head,-1,sizeof(head));
    lon=-1;
}
void edgemake(int from,int to,int c)
{
    e[++lon].c=c;
    e[lon].from=from;
    e[lon].to=to;
    e[lon].next=head[from];
    head[from]=lon;
}
void make(int from,int to,int c)
{
    edgemake(from,to,c);
    edgemake(to,from,0);
}

bool makelevel(int s,int t)
{
    memset(level,0,sizeof(level));
    int front=1,end=0;
    que[++end]=s;
    level[s]=1;
    while(front<=end)
    {
        int u=que[front++];
        if(u==t) return true;
        for(int k=head[u];k!=-1;k=e[k].next)
        {
            int v=e[k].to;
            if(!level[v]&&e[k].c)
            {
                que[++end]=v;
                level[v]=level[u]+1;
            }
        }
    }
    return false;
}

int dfs(int now,int t,int maxf)
{
    if(now==t||maxf==0) return maxf;
    int ret=0;
    for(int k=head[now];k!=-1;k=e[k].next)
    {
        int u=e[k].to;
        if(level[u]==level[now]+1&&e[k].c)
        {
            int f=dfs(u,t,min(e[k].c,maxf-ret));
            e[k].c-=f;
            e[k^1].c+=f;
            ret+=f;
            if(ret==maxf) return ret;
        }
    }
    if(ret==0) level[now]=0;
    return ret;
}

int maxflow(int s,int t)
{
    int ret=0;
    while(makelevel(s,t))
    {
        ret+=dfs(s,t,inf);
    }
    return ret;
}

void dfs1(int now)
{
    int i;
    from[now]=1;
    for(i=head[now];i!=-1;i=e[i].next)
        if(!from[e[i].to]&&e[i].c)
            dfs1(e[i].to);
}
void dfs2(int now)
{
    int i;
    to[now]=1;
    for(i=head[now];i!=-1;i=e[i].next)
        if(!to[e[i].to]&&e[i^1].c)
            dfs2(e[i].to);
}
int main()
{
    int s,t;
    while(1)
    {
        int i,j;
        int u,v,w;
        int flag=0;
        scanf("%d%d%d%d",&n,&m,&s,&t);
        if(n==0) break;
        edgeini();
        for(i=1;i<=m;i++)
        {
            scanf("%d%d%d",&u,&v,&w);
            make(u,v,w);
            make(v,u,w);
        }
        int flow=maxflow(s,t);
        memset(from,0,sizeof(from));
        memset(to,0,sizeof(to));
        dfs1(s);
        dfs2(t);
        for(i=1;i<=n;i++)
            if(from[i]==0&&to[i]==0)
                flag=1;
        if(flag) printf("AMBIGUOUS
");
        else printf("UNIQUE
");
    }
    return 0;
}