HDU 3452 Bonsai

#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>ljb[maxn];
bool vis[maxn];
int d[maxn];
int cur[maxn];
int flag[maxn];
int cost[maxn][maxn];
int n,m,s,t;

//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;
}

void dfs(int x,int fa)
{
if(fa!=-1)
AddEdge(fa,x,cost[fa][x]);
int FD=0;
for(int i=0;i<ljb[x].size();i++)
{
if(!flag[ljb[x][i]])
{
FD=1;
flag[ljb[x][i]]=1;
dfs(ljb[x][i],x);
}
}
if(FD==0) AddEdge(x,t,INF);
}

int main()
{
while(~scanf("%d%d",&n,&s))
{
if(n==0&&s==0) break;

t=n+1;
edges.clear();
for(int i=0;i<maxn;i++)
{
G[i].clear();
ljb[i].clear();
}

for(int i=1;i<=n-1;i++)
{
int uu,vv,cc;
scanf("%d%d%d",&uu,&vv,&cc);
ljb[uu].push_back(vv);
ljb[vv].push_back(uu);
cost[uu][vv]=cc;
cost[vv][uu]=cc;
}
memset(flag,0,sizeof(flag));
flag[s]=1;
dfs(s,-1);

int ans=Maxflow(s,t);
if(ans==INF) printf("0\n");
else printf("%d\n",ans);
}
return 0;
}