树分治算法

树分治

树分治有两种：

分治策略：

POJ 1741

#include <stdio.h>#include <string.h>#include <iostream>#include <algorithm>#include <vector>#include <queue>#include <set>#include <map>#include <string>#include <math.h>#include <stdlib.h>#include <time.h>using namespace std;const int MAXN = 10010;const int INF = 0x3f3f3f3f;struct Edge{    int to,next,w;}edge[MAXN*2];int head[MAXN],tot;void init(){    tot = 0;    memset(head,-1,sizeof(head));}void addedge(int u,int v,int w){    edge[tot].to = v; edge[tot].w = w;    edge[tot].next = head[u];head[u] = tot++;}bool vis[MAXN];int size[MAXN],dep[MAXN];int le,ri;int dfssize(int u,int pre){    size[u] = 1;    for(int i = head[u];i != -1;i = edge[i].next)    {        int v = edge[i].to;        if(v == pre || vis[v])continue;        size[u] += dfssize(v,u);    }    return size[u];}int minn;//找重心void getroot(int u,int pre,int totnum,int &root){    int maxx = totnum - size[u];    for(int i = head[u];i != -1;i = edge[i].next)    {        int v = edge[i].to;        if(v == pre || vis[v])continue;        getroot(v,u,totnum,root);        maxx = max(maxx,size[v]);    }    if(maxx < minn){minn = maxx; root = u;}}void dfsdepth(int u,int pre,int d){    dep[ri++] = d;    for(int i = head[u];i != -1;i = edge[i].next)    {        int v = edge[i].to;        if(v == pre || vis[v])continue;        dfsdepth(v,u,d+edge[i].w);    }}int k;int getdep(int a,int b){    sort(dep+a,dep+b);    int ret = 0, e = b-1;    for(int i = a;i < b;i++)    {        if(dep[i] > k)break;        while(e >= a && dep[e] + dep[i] > k)e--;        ret += e - a + 1;        if(e > i)ret--;    }    return ret>>1;}int solve(int u){    int totnum = dfssize(u,-1);    int ret = 0;    minn = INF;    int root;    getroot(u,-1,totnum,root);    vis[root] = true;    for(int i = head[root];i != -1;i = edge[i].next)    {        int v = edge[i].to;        if(vis[v])continue;        ret += solve(v);     }    le = ri = 0;    for(int i = head[root];i != -1;i = edge[i].next)    {        int v = edge[i].to;        if(vis[v])continue;        dfsdepth(v,root,edge[i].w);        ret -= getdep(le,ri);        le = ri;    }    ret += getdep(0,ri);    for(int i = 0;i < ri;i++)    {        if(dep[i] <= k)ret++;        else break;    }    vis[root] = false;    return ret;}int main(){    //freopen("in.txt","r",stdin);    //freopen("out.txt","w",stdout);    int n;    int u,v,w;    while(scanf("%d%d",&n,&k) == 2)    {        if(n == 0 && k == 0)break;        init();        for(int i = 1;i < n;i++)        {            scanf("%d%d%d",&u,&v,&w);            addedge(u,v,w);            addedge(v,u,w);        }        memset(vis,false,sizeof(vis));        printf("%d\n",solve(1));    }    return 0;}