bzoj 3420: Poi2013 Triumphal arch binary tree dp +

code:

#include <bits/stdc++.h>      
#define N 300005   
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;  
int n,edges,k;        
int hd[N],to[N<<1],nex[N<<1],f[N];        
void add(int u,int v) 
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v;   
}
void dfs(int u,int ff) 
{
    int tot=0;      
    f[u]=-k;            
    for(int i=hd[u];i;i=nex[i]) 
    {
        int v=to[i]; 
        if(v==ff) continue;   
        dfs(v,u);   
        f[u]+=f[v]+1;   
    }   
    // printf("%d %d\n",u,f[u]);    
    f[u]=max(f[u], 0);   
}
int main() 
{ 
    // setIO("input"); 
    int i,j; 
    scanf("%d",&n);  
    for(i=1;i<n;++i) 
    {
        int u,v; 
        scanf("%d%d",&u,&v),add(u,v),add(v,u);   
    }
    int l=0,r=n,ans=n;   
    while(l<=r) 
    {
        int mid=(l+r)>>1;     
        for(i=1;i<=n;++i)  f[i]=0;   
        k=mid, dfs(1,0);    
        if(f[1]==0)   ans=mid, r=mid-1;  
        else    l=mid+1; 
    }
    printf("%d\n",ans);    
    return 0;   
}