点分治是一种处理树的优秀暴力

这是一道板子题

#include <cstdio>

#include <cstring>

#include <algorithm>

using namespace std;

int u[50010<<1],v[50010<<1],fir[50010],nxt[50010<<1],cnt,root,sz[50010],f[50010],middis[50010],midcnt,n,k,vis[50010],Siz;

long long ans=0;

void addedge(int ui,int vi){

    ++cnt;

    u[cnt]=ui;

    v[cnt]=vi;

    nxt[cnt]=fir[ui];

    fir[ui]=cnt;

}

void getroot(int u,int fa){

    sz[u]=1,f[u]=1;

    for(int i=fir[u];i;i=nxt[i]){

        if(v[i]==fa||vis[v[i]])

            continue;

        getroot(v[i],u);

        sz[u]+=sz[v[i]];

        f[u]=max(f[u],sz[v[i]]);

    }

    f[u]=max(Siz-sz[u],f[u]);

    if(f[u]<f[root])

        root=u;

}

void getdis(int u,int d,int fa){

    // printf("ux=%d\n",u);

    middis[++midcnt]=d;

    for(int i=fir[u];i;i=nxt[i]){

        if(vis[v[i]]||v[i]==fa)

            continue;

        getdis(v[i],d+1,u);

    }

}

int look1(int l,int k){

    int ans=0,r=midcnt;

    while(l<=r){

        int mid=(l+r)>>1;

        if(middis[mid]<k)

            l=mid+1;

        else

            ans=mid,r=mid-1;

    }

    return ans;

}

int look2(int l,int k){

    int ans=0,r=midcnt;

    while(l<=r){

        int mid=(l+r)>>1;

        if(middis[mid]<=k)

            l=mid+1,ans=mid;

        else

            r=mid-1;

    }

    return ans;

}

int solve(void){

    sort(middis+1,middis+midcnt+1);

    // for(int i=1;i<=midcnt;i++)

    //     printf("%d ",middis[i]);

    // getchar();

    // printf("\n");

    int mid=0;

    int l=1;

    while(l<midcnt&&middis[l]+middis[midcnt]<k)

        ++l;

    while(l<midcnt&&k-middis[l]>=middis[l]){

        int l2=look2(l+1,k-middis[l]),l1=look1(l+1,k-middis[l]);

        if(l2>=l1)

            mid+=l2-l1+1;

        l++;

    }

    return mid;

}

void divide(int u){

    // printf("u=%d\n",u);

    // getchar();

    vis[u]=true;

    midcnt=0;

    getdis(u,0,0);

    // printf("ok\n");

    ans+=solve();

    // printf("an=%d\n",ans);

    for(int i=fir[u];i;i=nxt[i]){

        if(vis[v[i]])

            continue;

        midcnt=0;

        getdis(v[i],1,0);

        ans-=solve();

        // printf("s=%d\n",ans);

        root=0;

        Siz=sz[v[i]];

        getroot(v[i],u);

        divide(root);

    }

}

int main(){

    scanf("%d %d",&n,&k);

    for(int i=1;i<=n-1;i++){

        int a,b;

        scanf("%d %d",&a,&b);

        addedge(a,b);

        addedge(b,a);

    }

    Siz=n;

    f[0]=0x3f3f3f3f;

    getroot(1,0);

    divide(root);

    printf("%lld",ans);

    return 0;

}