Codeforces 161D Distance in Tree

题目大意：给出一棵n个节点的树，统计树中长度为k的路径的条数（1<=n<=50000 , 1<=k<=500）

思路:树分治！

#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<iostream>
#define ll long long
int n,K,son[],F[],sum;
ll ans;
int c[],pd[],vis[],A[];
int sz,dis[],root,mxdeep,deep[];
int tot,go[],first[],next[];
void insert(int x,int y){
    tot++;
    go[tot]=y;
    next[tot]=first[x];
    first[x]=tot;
}
void add(int x,int y){
    insert(x,y);insert(y,x);
}
int read(){
    int t=,f=;char ch=getchar();
    while (ch<''||ch>''){if (ch=='-') f=-;ch=getchar();}
    while (''<=ch&&ch<=''){t=t*+ch-'';ch=getchar();}
    return t*f;
}
void findroot(int x,int fa){
    son[x]=;F[x]=;
    for (int i=first[x];i;i=next[i]){
        int pur=go[i];
        if (pur==fa||vis[pur]) continue;
        findroot(pur,x);
        F[x]=std::max(F[x],son[pur]);
        son[x]+=son[pur];
    }
    F[x]=std::max(F[x],sum-son[x]);
    if (F[x]<F[root]) root=x;
}
void Dfs(int x,int fa){
    mxdeep=std::max(mxdeep,deep[x]);
    for (int i=first[x];i;i=next[i]){
        int pur=go[i];
        if (fa==pur||vis[pur]) continue;
        deep[pur]=deep[x]+;
        Dfs(pur,x);
    }
}
void bfs(int x){
    pd[x]=sz;
    int h=,t=;c[]=x;dis[x]=;
    while (h<=t){
        int now=c[h++];
        for (int i=first[now];i;i=next[i]){
            int pur=go[i];
            if (pd[pur]==sz||vis[pur]) continue;
            dis[pur]=dis[now]+;
            c[++t]=pur;
            pd[pur]=sz;
        }
    }
    for (int i=;i<=t;i++)
     if (K>=dis[c[i]])
     ans+=A[K-dis[c[i]]];
    for (int i=;i<=t;i++)
     A[dis[c[i]]]++;
}
void solve(int x,int fa){
    vis[x]=;
    mxdeep=;
    deep[x]=;
    Dfs(x,);
    for (int i=;i<=mxdeep;i++)
     A[i]=;
    A[]=;
    sz++;
    for (int i=first[x];i;i=next[i]){
        int pur=go[i];
        if (pur==fa||vis[pur]) continue;
        bfs(pur);
    }
    int cnt=sum;
    for (int i=;i<=mxdeep;i++)
     A[i]=;
    for (int i=first[x];i;i=next[i]){
        int pur=go[i];
        if (pur==fa||vis[pur]) continue;
        root=;
        if (son[pur]>son[x]) sum=cnt-son[x];
        else sum=son[pur];
        findroot(pur,x);
        solve(root,x);
    }
}
int main(){
    n=read();K=read();
    for (int i=;i<n;i++){
        int x=read(),y=read();
        add(x,y);
    }
    F[]=0x7fffffff;root=;sum=n;
    findroot(,);
    solve(root,);
    printf("%I64d\n",ans);
}