題目鏈接：Tree

樹分治參考資料：09年漆子超 ZigZagK的博客 【點分治】的學習筆記和眾多例題

以上的資料是我看過好的，具體算法分析實現都講明白，貼一下我的代碼（由於poj交不了題還沒a）

 1 #include<bits/stdc++.h>
 2 #include<set>
 3 #include<cstdio>
 4 #include<iomanip>
 5 #include<iostream>
 6 #include<string>
 7 #include<cstring>
 8
 
 #include<algorithm>
 9 #define pb push_back
10 #define ll long long
11 #define fi first
12 #define se second
13 #define PI 3.14159265
14 #define ls l,m,rt<<1
15 #define rs m+1,r,rt<<1|1
16 #define eps 1e-7
17 #define pii pair<int,int>
18 typedef unsigned long long ull;
 
19 const int mod=1e3+5;
20 const ll inf=0x3f3f3f3f3f3f3f;
21 const int maxn=1e5+5;
22 using namespace std;
23 int n,m,head[maxn],to[maxn*2],w[maxn*2],nxt[maxn*2],cnt,k,siz[maxn],dep[maxn],rt,mx[maxn],s;
24 int now[maxn];
25 bool vis[maxn];
26 ll ans;
27 void add_edge(int x,int y,int l)
28 {
 
29     to[++cnt]=y;w[cnt]=l;nxt[cnt]=head[x];head[x]=cnt;
30 }
31 void dfs_rt(int v,int f)//找重心
32 {
33     siz[v]=1;mx[v]=0;
34     //cout<<v<<"SSS"<<f<<endl;
35     for(int i=head[v];i;i=nxt[i])
36     {
37         if(to[i]==f||vis[to[i]])continue;
38         dfs_rt(to[i],v);
39         siz[v]+=siz[to[i]];
40         mx[v]=max(siz[to[i]],mx[v]);
41     }
42     mx[v]=max(s-siz[v],mx[v]);
43     if(rt==-1||mx[v]<mx[rt])rt=v;
44 }
45 void get_dep(int v,int f)
46 {
47     now[++now[0]]=dep[v];
48     for(int i=head[v];i;i=nxt[i])
49     {
50         if(vis[to[i]]||to[i]==f)continue;
51         dep[to[i]]=dep[v]+w[i];
52         get_dep(to[i],v);
53     }
54 }
55 int  get_sum(int v,int dis)
56 {
57     int cn=0;now[0]=0;
58     dep[v]=dis;get_dep(v,-1);
59     sort(now+1,now+1+now[0]);
60     int l=1,r=now[0];
61     while(l<r)if(now[l]+now[r]<=k)cn+=r-l,l++;else r--;
62     return cn;
63 }
64 void get_ans(int v)
65 {
66     vis[v]=true;ans+=get_sum(v,0);
67     cout<<"###"<<ans<<endl;
68     for(int i=head[v];i;i=nxt[i])
69     {
70         if(vis[to[i]])continue;
71         ans-=get_sum(to[i],w[i]);
72          cout<<"SSS"<<ans<<endl;
73         rt=-1;s=siz[to[i]];dfs_rt(to[i],v);
74         get_ans(rt);
75     }
76 }
77 int main()
78 {
79     while(~scanf("%d %d",&n,&k))
80     {
81         if(!n&&!k)break;
82         memset(head,0,sizeof(head));cnt=0;
83         memset(vis,0,sizeof(vis));
84         for(int i=0;i<n-1;i++)
85         {
86             int u,v,l;
87             scanf("%d %d %d",&u,&v,&l);
88             add_edge(u,v,l);add_edge(v,u,l);
89         }
90         rt=-1;s=n;dfs_rt(1,-1);
91         get_ans(rt);
92         printf("%lld\n",ans);
93     }
94     return 0;
95 }

第二個例題：2152: 聰聰可可

題解跟上面那題差不多，計算出的到根的距離%3之後的值的個數，now[0],now[1],now[2].答案就是now[0]*now[0]+2*now[1]*now[2]

 1 #include<bits/stdc++.h>
 2 #include<set>
 3 #include<cstdio>
 4 #include<iomanip>
 5 #include<iostream>
 6 #include<string>
 7 #include<cstring>
 8 #include<algorithm>
 9 #define pb push_back
10 #define ll long long
11 #define fi first
12 #define se second
13 #define PI 3.14159265
14 #define ls l,m,rt<<1
15 #define rs m+1,r,rt<<1|1
16 #define eps 1e-7
17 #define pii pair<int,int>
18 typedef unsigned long long ull;
19 const int mod=1e3+5;
20 const ll inf=0x3f3f3f3f3f3f3f;
21 const int maxn=2e4+5;
22 using namespace std;
23 int n,m,head[maxn],to[maxn*2],w[maxn*2],nxt[maxn*2],cnt,k,siz[maxn],dep[maxn],rt,mx[maxn],s;
24 int now[maxn];
25 bool vis[maxn];
26 int ans;
27 void add_edge(int x,int y,int l)
28 {
29     to[++cnt]=y;w[cnt]=l;nxt[cnt]=head[x];head[x]=cnt;
30 }
31 void dfs_rt(int v,int f)//找重心
32 {
33     siz[v]=1;mx[v]=0;
34     for(int i=head[v];i;i=nxt[i])
35     {
36         if(to[i]==f||vis[to[i]])continue;
37         dfs_rt(to[i],v);
38         siz[v]+=siz[to[i]];
39         mx[v]=max(siz[to[i]],mx[v]);
40     }
41     mx[v]=max(s-siz[v],mx[v]);
42     if(rt==-1||mx[v]<mx[rt])rt=v;
43 }
44 void get_dep(int v,int f)
45 {
46     now[(dep[v])%3]++;
47     for(int i=head[v];i;i=nxt[i])
48     {
49         if(vis[to[i]]||to[i]==f)continue;
50         dep[to[i]]=dep[v]+w[i];
51         get_dep(to[i],v);
52     }
53 }
54 int  get_sum(int v,int dis)
55 {
56     int cn=0;now[0]=0,now[1]=0,now[2]=0;
57     dep[v]=dis;get_dep(v,-1);
58     cn+=now[0]*now[0]+2*now[1]*now[2];
59     return cn;
60 }
61 void get_ans(int v)
62 {
63     vis[v]=true;ans+=get_sum(v,0);
64     for(int i=head[v];i;i=nxt[i])
65     {
66         if(vis[to[i]])continue;
67         ans-=get_sum(to[i],w[i]);
68         rt=-1;s=siz[to[i]];dfs_rt(to[i],v);
69         get_ans(rt);
70     }
71 }
72 int main()
73 {
74     while(~scanf("%d",&n))
75     {
76         memset(head,0,sizeof(head));cnt=0;
77         memset(vis,0,sizeof(vis));
78         for(int i=0;i<n-1;i++)
79         {
80             int u,v,l;
81             scanf("%d %d %d",&u,&v,&l);
82             add_edge(u,v,l);add_edge(v,u,l);
83         }
84         rt=-1;s=n;dfs_rt(1,-1);
85         get_ans(rt);
86         int sum=n*n;
87         int tmp=__gcd(sum,ans);
88 
89         ans/=tmp;sum/=tmp;
90         printf("%d/%d\n",ans,sum);
91     }
92     return 0;
93 }

樹分治入門：例題poj1741