[bzoj2097][Usaco2010 Dec]Exercise 奶牛健美操_贪心_树形dp_二分

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define N 100010 
using namespace std;
inline char nc() {static char *p1,*p2,buf[100000]; return (p1==p2)&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;}
int rd() {int x=0; char c=nc(); while(!isdigit(c)) c=nc(); while(isdigit(c)) x=(x<<3)+(x<<1)+(c^48),c=nc(); return x;}
int to[N<<1],nxt[N<<1],head[N],tot;
int cnt,max_dis[N],a[N];
inline void add(int x,int y)
{
	to[++tot]=y;
	nxt[tot]=head[x];
	head[x]=tot;
}
inline bool cmp(int x,int y) {return x>y;}
void dfs(int pos,int fa,int limit)
{
	bool flag=false;
	max_dis[pos]=0;
	for(int i=head[pos];i;i=nxt[i])
	{
		if(to[i]==fa) continue;
		flag=true;
		dfs(to[i],pos,limit);
		max_dis[pos]=max(max_dis[pos],max_dis[to[i]]+1);
	}
	if(!flag) {max_dis[pos]=0; return;}
	a[0]=0;
	for(int i=head[pos];i;i=nxt[i])
	{
		if(to[i]==fa) continue;
		a[++a[0]]=max_dis[to[i]]+1;
	}
	sort(a+1,a+a[0]+1,cmp);
	for(int i=1;i<a[0];i++)
	{
		if(a[i]+a[i+1]>limit) cnt++,a[i]=0;
	}
	if(a[a[0]]>limit) cnt++,a[a[0]]=0;
	sort(a+1,a+a[0]+1,cmp);
	max_dis[pos]=a[1];
}
int n,m;
int calc(int limit)
{
	cnt=0;
	dfs(1,0,limit);
	return cnt;
}
int find()
{
	int l=0,r=n-1,ans=0;
	while(l<=r)
	{
		int mid=(l+r)>>1;
		if(calc(mid)<=m) ans=mid,r=mid-1;
		else l=mid+1;
	}
	return ans;
}
int main()
{
	n=rd(),m=rd();
	int x,y;
	for(int i=1;i<n;i++)
	{
		x=rd(),y=rd();
		add(x,y),add(y,x);
	}
	printf("%d\n",find());
}