poj 1741 Tree 树上的分治

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http://poj.org/problem?id=1741

算法：

寻找树的重心；

分治；

#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>using namespace std;const int maxn = 10010;int n , m , k;int size[maxn] , mx[maxn] , mi , dis[maxn] , center , cnt , ans;bool vis[maxn];struct Edge {int v , w , next;Edge() {}Edge(int v,int w,int next) : v(v) , w(w) , next(next) {};}edge[maxn<<1];int E , head[maxn];void init() {E = 0;memset(head,-1,sizeof(int)*(n+1));}void addedge(int u,int v,int w) {edge[E] = Edge(v,w,head[u]); head[u] = E++;edge[E] = Edge(u,w,head[v]); head[v] = E++;}void dfssize(int u,int fa) {size[u] = 1;mx[u] = 0;for(int i=head[u];i!=-1;i=edge[i].next) {int v = edge[i].v;if(vis[v] || v == fa) continue;dfssize(v , u);size[u] += size[v];if(size[v] > mx[u]) mx[u] = size[v];}return;}void dfscenter(int r,int u,int fa) {int tmp = max(mx[u] , size[r]-size[u]);if(tmp < mi) mi = tmp , center = u;for(int i=head[u];i!=-1;i=edge[i].next) {int v = edge[i].v;if(vis[v] || v == fa) continue;dfscenter(r,v,u);}return;}void dfsdis(int u,int d,int fa) {dis[cnt++] = d;for(int i=head[u];i!=-1;i=edge[i].next) {int v = edge[i].v;if(vis[v] || v == fa) continue;dfsdis(v,d+edge[i].w,u);}return;}int calc(int u,int d) {int ret = 0;cnt = 0;dfsdis(u,d,-1);sort(dis,dis+cnt);int i = 0 , j = cnt - 1;while(i < j) {while(i<j & dis[i]+dis[j]>k) j--;ret += j-i;i++;}return ret;}void dfs(int u) {mi = n;dfssize(u,-1);dfscenter(u,u,-1);ans += calc(center,0);vis[center] = true;for(int i=head[center];i!=-1;i=edge[i].next) {int v = edge[i].v;if(vis[v]) continue;ans -= calc(v,edge[i].w);dfs(v);}}int main() {while(~scanf("%d%d" , &n,&k) && n+k) {init();memset(vis,false,sizeof(bool)*(n+1));ans = 0;for(int i=1;i<n;i++) {int u ,v , w;scanf("%d%d%d" ,&u,&v,&w);addedge(u,v,w);}dfs(1);printf("%d\n" , ans);}return 0;}

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