考试题 : 游戏 (二分，贪心)

Code

#include<bits/stdc++.h>
#define N 2000008
using namespace std;

struct sj{
    int to,next;
}a[N*2];
int size,head[N],n,k;

void in(int &x)
{
    char ch=getchar();int f=1,w=0;
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){w=w*10+ch-'0';ch=getchar();}
    x=f*w; return;
}

void add(int x,int y)
{
    a[++size].to=y;
    a[size].next=head[x];
    head[x]=size;
}

int dep[N],dst[N],st,res;
void dfs(int fr,int x)
{
    dst[x]=dep[x]=dep[fr]+1; 
    int lm,rm;
    lm=0; rm=19260817;
    int ljs=0,rjs=0;
    for(int i=head[x];i;i=a[i].next)
    {
        int tt=a[i].to;
        if(tt==fr)continue;
        dfs(x,tt);
        if(dst[tt]-dep[x]>(st/2))
            rm=min(rm,dst[tt]-dep[x]),rjs++;
        else 
            lm=max(lm,dst[tt]-dep[x]),ljs++;
    }
    if(rm+lm>st)
        res-=rjs,dst[x]=dep[x]+lm;
    else 
        res-=(rjs-1),dst[x]=dep[x]+rm;
    return;
}

bool judge()
{
    dfs(-1,1);
    return (res>=0);    
}

int main()
{
	freopen("game.in","r",stdin);
	freopen("game.out","w",stdout);
    in(n),in(k);
    for(int i=1;i<n;i++)
    {
        int x,y; 
        in(x),in(y);
        add(x,y);add(y,x);
    }
    int l=0,r=n,ans=n*2;
    while(l<=r)
    {
        int mid=(l+r)/2; st=mid; res=k;
        if(judge())r=mid-1,ans=min(ans,mid);
        else l=mid+1;
    }
    cout<<ans<<endl;
    return 0;
}