BZOJ 2097: [Usaco2010 Dec]Exercise 奶牛健美操 二分 + 贪心 + 树上问题

Code: 

#include<bits/stdc++.h>
using namespace std; 
#define setIO(s) freopen(s".in","r",stdin) 
#define maxn 1000000  
int n,S,edges,mid,cnt; 
int hd[maxn],to[maxn<<1],nex[maxn<<1],dis[maxn];    
void add(int u,int v) 
{
    nex[++edges]=hd[u],hd[u]=edges,to[edges]=v; 
}  
void dfs(int u,int ff) 
{  
    if(cnt>S) return; 
    int now=0; 
    for(int i=hd[u];i;i=nex[i]) 
    {       
        int v=to[i]; 
        if(v==ff) continue; 
        dfs(v,u); 
        if(now+dis[v] > mid) 
        {
            ++cnt;             
            now=min(now, dis[v]); 
        }
        else now=max(now, dis[v]); 
    }
    dis[u]=now+1; 
}
int check() 
{  
    cnt=0; 
    dfs(1,0);     
    return cnt<=S;   
}
int main()
{
    // setIO("input"); 
    int i,j; 
    scanf("%d%d",&n,&S); 
    for(i=1;i<n;++i) 
    {
        int a,b; 
        scanf("%d%d",&a,&b); 
        add(a,b),add(b,a); 
    } 
    int l=1,r=n,ans=0; 
    while(l<=r) 
    {
        mid=(l+r)>>1; 
        if(check()) ans=mid, r=mid-1; 
        else l=mid+1; 
    } 
    printf("%d\n",ans);   
    return 0; 
}