[bzoj 2097]奶牛健美操

#include<map>
#include<stack>
#include<queue>
#include<cstdio>
#include<string>
#include<vector>
#include<cstring>
#include<complex>
#include<iostream>
#include<assert.h>
#include<algorithm>
using namespace std;
#define inf 1001001001
#define infll 1001001001001001001LL
#define ll long long
#define dbg(vari) cerr<<#vari<<" = "<<(vari)<<endl
#define gmax(a,b) (a)=max((a),(b))
#define gmin(a,b) (a)=min((a),(b))
#define Ri register int
#define gc getchar()
#define il inline
il int read(){
    bool f=true;Ri x=0;char ch;while(!isdigit(ch=gc))if(ch==‘-‘)f=false;while(isdigit(ch)){x=(x<<1)+(x<<3)+ch-‘0‘;ch=gc;}return f?x:-x;
}
#define gi read()
#define FO(x) freopen(#x".in","r",stdin),freopen(#x".out","w",stdout);
struct edge{
    int to,next;
}e[233333];
int last[100005],ecnt,cnt,n,k,lb,rb,mid,ans;
int f[100005],_a,a[100005];
int dfs(int x,int fa=0){
    f[x]=0;
    //...
    for(int i=last[x];i;i=e[i].next)if(e[i].to!=fa)dfs(e[i].to,x);
    _a=a[0]=0;
    for(int i=last[x];i;i=e[i].next)if(e[i].to!=fa)a[++_a]=f[e[i].to]+1;
    sort(a+1,a+_a+1);
    while(_a&&a[_a]+a[_a-1]>mid)_a--,ans++;
    f[x]=a[_a];
}
bool chk(){
    ans=0;
    dfs(1);
    return ans<=k;
}
int main(){
    n=gi;k=gi;
    for(int i=1;i<n;i++){
        int a=gi,b=gi;
        e[++ecnt]=(edge){b,last[a]};last[a]=ecnt;
        e[++ecnt]=(edge){a,last[b]};last[b]=ecnt;        
    }
    int ans=n,lb=1,rb=n;
    while(lb<=rb){
        mid=(lb+rb)/2;
        if(chk()){
            ans=mid;
            rb=mid-1;
        }else{
            lb=mid+1;
        }
    }
    printf("%d",ans);
    return 0;
}