POJ 1741 Tree ——（树分治）

  1 #include <stdio.h>
  2 #include <algorithm>
  3 #include <string.h>
  4 #include <vector>
  5 using namespace std;
  6 const int N = 10000 + 5;
  7 
  8 struct edge
  9 {
 10     int v,w;
 11 };
 12 int n,k,root,sz,ans;
 13 bool vis[N];
 14 int son[N],f[N],d[N];
 15 vector<edge> G[N];
 16 vector<int> dep;
 17 void init()
 18 {
 19     memset(vis,false,sizeof(vis));
 20     for(int i=1;i<=n;i++) G[i].clear();
 21     memset(son,0,sizeof son);
 22     memset(f,0,sizeof f);
 23 }
 24 void  addEdge(int u,int v,int w)
 25 {
 26     G[u].push_back((edge){v,w});
 27     G[v].push_back((edge){u,w});
 28 }
 29 void getRoot(int u,int fa)
 30 {
 31     son[u] = 1; f[u] = 0;
 32     for(int i=0;i<G[u].size();i++)
 33     {
 34         edge e = G[u][i];
 35         int v = e.v;
 36         if(v == fa || vis[v]) continue;
 37         getRoot(v,u);
 38         son[u] += son[v];
 39         f[u] = max(f[u], son[v]);
 40     }
 41     f[u] = max(f[u], sz-son[u]);
 42     if(f[u] < f[root]) root = u;
 43 }
 44 void getDep(int u,int fa)
 45 {
 46     dep.push_back(d[u]);
 47     for(int i=0;i<G[u].size();i++)
 48     {
 49         edge e = G[u][i];
 50         int v = e.v, w= e.w;
 51         if(v == fa || vis[v]) continue;
 52         d[v] = d[u] + w;
 53         getDep(v,u);
 54     }
 55 }
 56 int cal(int u,int len)
 57 {
 58     dep.clear(); d[u] = len;
 59     getDep(u, 0);
 60     sort(dep.begin(), dep.end());
 61     int ret = 0;
 62     for(int l=0,r=dep.size()-1;l<r;)
 63     {
 64         if(dep[l] + dep[r] <= k) ret += r - (l++);
 65         else r--;
 66     }
 67     return ret;
 68 }
 69 void solve(int u)
 70 {
 71     ans += cal(u, 0);
 72     vis[u] = 1;
 73     for(int i=0;i<G[u].size();i++)
 74     {
 75         edge e = G[u][i];
 76         int v = e.v, w = e.w;
 77         if(vis[v]) continue;
 78         ans -= cal(v, w);
 79         f[0] = sz = son[v];
 80         getRoot(v, root=0);
 81         solve(root);
 82     }
 83 }
 84 
 85 int main()
 86 {
 87     while(scanf("%d%d",&n,&k) == 2)
 88     {
 89         if(n == 0 && k == 0) break;
 90         init();
 91         for(int i=1;i<=n-1;i++)
 92         {
 93             int u,v,w;
 94             scanf("%d%d%d",&u,&v,&w);
 95             addEdge(u,v,w);
 96         }
 97         f[0] = sz = n;
 98         getRoot(1,root=0);
 99         ans = 0;
100         solve(root);
101         printf("%d\n",ans);
102     }
103     return 0;
104 }