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;
}