HDU - 3452 Bonsai 最小割

给出一棵无向树，要求把根节点和叶子节点分割开的最小权值

最大流最小割板子题，源点题目中已经给出，需要添加一个汇点。再添加叶子节点与汇点的边，权值为INF

#include<bits/stdc++.h>
using namespace std;
const int N=3234;
const int M=1123456;
const int INF=0x3f3f3f3f;
int n,m;
struct node
{
int u,v,next,cap;
} eage[M];
int source,sink;
int cur
,dep
,gap
;
int head
;
int deg
;
int top;
int S
;
void Add(int u,int v,int w)
{
eage[top].u=u;
eage[top].v=v;
eage[top].cap=w;
eage[top].next=head[u];
head[u]=top++;

eage[top].u=v;
eage[top].v=u;
eage[top].cap=0;
eage[top].next=head[v];
head[v]=top++;
}
void BFS()
{
queue<int>q;
memset(dep,-1,sizeof(dep));
memset(gap,0,sizeof(gap));
gap[0]=1;
dep[sink]=0;
q.push(sink);
while(!q.empty())
{
int u=q.front();
q.pop();
for(int i=head[u]; i!=-1; i=eage[i].next)
{
int v=eage[i].v;
if(dep[v]==-1)
{
q.push(v);
dep[v]=dep[u]+1;
gap[dep[v]]++;
}
}
}
}
int Sap()
{
BFS();
memcpy(cur,head,sizeof(head));
int tot=0;
int u=source;
int ans=0;
while(dep[source]<n)
{
if(u==sink)
{
int Min=INF;
int inser;
for(int i=0; i<=tot-1; i++)
{
if(Min>eage[S[i]].cap)
{
Min=eage[S[i]].cap;
inser=i;
}
}
for(int i=0; i<=tot-1; i++)
{
eage[S[i]].cap-=Min;
eage[S[i]^1].cap+=Min;
}
ans+=Min;
tot=inser;
u=eage[S[tot]].u;
}
if(u!=sink&&gap[dep[u]-1]==0)
break;
int v;
int i;
for(i=cur[u]; i!=-1; i=eage[i].next)
{
v=eage[i].v;
if(eage[i].cap&&dep[v]+1==dep[u])
{
break;
}
}
if(i!=-1)
{
cur[u]=i;
S[tot++]=i;
u=v;
continue;
}
int Min=n;
for(int i=head[u]; i!=-1; i=eage[i].next)
{
int v=eage[i].v;
if(eage[i].cap&&dep[v]<Min)
{
Min=dep[v];
cur[u]=i;
}
}
gap[dep[u]]--;
dep[u]=Min+1;
gap[dep[u]]++;
if(u!=source)u=eage[S[--tot]].u;
}
return ans;
}

int main()
{
while(~scanf("%d%d",&n,&source))
{
if(n==0&&source==0)break;
memset(deg,0,sizeof(deg));
memset(head,-1,sizeof(head));
top=0;
sink=n+1;
for(int i=1;i<=n-1;i++)
{
int u,v,w;
scanf("%d%d%d",&u,&v,&w);
deg[u]++;
deg[v]++;
Add(u,v,w);
Add(v,u,w);
}
for(int i=1;i<=n;i++)
{
if(deg[i]==1&i!=source)
{
Add(i,sink,INF);
}
}
n=sink+1;
printf("%d\n",Sap());
}
return 0;
}