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先两次dfs找树的直径

emmmm, n \le 300 !!!

随便写好了

原来bzoj还有一道加强版,双倍经验多好

一定要在直径上取,而且距离不可以大于s,那么单调队列的思想不是可以吗?

因为离直径中点越近“贡献越大”

意会一下QwQ

这样下来代码量也很短 = ̄ω ̄=

而且可以ac bzoj的加强版

#include<bits/stdc++.h>
namespace ZDY{
    #pragma GCC optimize(3)
    #define il __inline__ __attribute__ ((always_inline))
    #define rg register
    #define ll long long
    #define ull unsigned long long
    #define db double
    #define sht short
    #define MB template <class T>il
    #define Fur(i,x,y) for(int i=x;i<=y;i++)
    #define Fdr(i,x,y) for(int i=x;i>=y;i--)
    #define fl(i,x) for(int i=head[x],to;to=e[i].to,i;i=e[i].nxt)
    #define clr(x,y) memset(x,y,sizeof(x))
    #define cpy(x,y) memcpy(x,y,sizeof(x))
    #define fin(s) freopen(s".in","r",stdin)
    #define fout(s) freopen(s".out","w",stdout)
    #define fcin ios::sync_with_stdio(false)
    #define l2(n) (int(log2(n)))
    #define inf 0x3f3f3f3f
    MB T ABS(T x){return x>0?x:-x;}
    MB T MAX(T x,T y){return x>y?x:y;}
    MB T MIN(T x,T y){return x<y?x:y;}
    MB T GCD(T x,T y){return y?GCD(y,x%y):x;}
    MB void SWAP(T &x,T &y){T t=x;x=y;y=t;}
}using namespace ZDY;using namespace std;
namespace IO{const char* ln="\n";const int str=1<<20;struct IN{char buf[str],*s,*t;bool _;IN():s(buf),t(buf),_(0){}il char gc(){return s==t&&((t=(s=buf)+fread(buf,1,str,stdin))==s)?EOF:(*s++);}IN&operator>>(char&ch){if(_)return *this;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF)_=1;else ch=c;return *this;}IN& operator>>(char* ch){if(_)return *this;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF)return _=1,*this;*ch=c;ch++;while((c=gc())!=EOF&&!isspace(c))*ch=c,ch++;if(c==EOF)_=1;return *this;}IN& operator>>(string& ch){if(_)return *this;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF)return _=1,*this;ch+=c;while((c=gc())!=EOF&&!isspace(c))ch+=c;if(c==EOF)_=1;return *this;}template<typename T>IN&operator>>(T&x){if(_)return *this;char c=gc();bool ff=0;while(c!=EOF&&(c<'0'||c>'9'))ff^=(c=='-'),c=gc();if(c==EOF){_=1;return *this;}x=0;while(c!=EOF&&'0'<=c&&c<='9')x=(x<<3)+(x<<1)+c-48,c=gc();if(c==EOF)_=1;if(ff)x=-x;return *this;}}in;struct OUT{char buf[str],*s,*t;OUT():s(buf),t(buf+str){}~OUT(){fwrite(buf,1,s-buf,stdout);}void pt(char c){(s==t)?(fwrite(s=buf,1,str,stdout),*s++=c):(*s++=c);}OUT&operator<<(const char*s){while(*s)pt(*s++);return *this;}OUT&operator<<(char*s){while(*s)pt(*s++);return *this;}OUT&operator<<(string s){for(int i=0;s[i];i++)pt(s[i]);return *this;}template<typename T>OUT&operator<<(T x){if(!x)return pt('0'),*this;if(x<0)pt('-'),x=-x;char a[30],t=0;while(x)a[t++]=x%10,x/=10;while(t--)pt(a[t]+'0');return *this;}}out;}using namespace IO;
#define N 500011
int n,s,cnt=0,head[N],d[N],f[N],tmp=0,top,ans=inf;
bool b[N];
struct edge{
    int to,nxt,w;
}e[N*2];
void add(int x,int y,int w){
    e[++cnt].to=y;e[cnt].w=w;e[cnt].nxt=head[x];head[x]=cnt;
}
void dfs(int x){
    if(d[x]>d[tmp])tmp=x;
    fl(i,x)
    if(to!=f[x]&&!b[to])d[to]=d[x]+e[i].w,f[to]=x,dfs(to);
}
int main(){
    in>>n>>s;
    int x,y,w;
    Fur(i,1,n-1)
        in>>x>>y>>w,
        add(x,y,w),add(y,x,w);

    d[1]=1;dfs(1);
    d[tmp]=0;f[tmp]=0;
    dfs(tmp);
    top=tmp;
    int h=0,t=1;
    for(int i=top,j=top;i;i=f[i]){
        while(d[j]-d[i]>s)j=f[j];
        ans=MIN(ans,MAX(d[top]-d[j],d[i]));
    }
    for(int i=top;i;i=f[i])b[i]=1;
    for(int i=top;i;i=f[i]){
        x=i;d[i]=0;
        dfs(i);
    }
    Fur(i,1,n)ans=MAX(ans,d[i]);
    out<<ans<<ln;
}
LG 1099 树网的核
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