[CF911F]Tree Destruction

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<queue>
 4 #include<cstring>
 5 #define M 200010
 6 using namespace std;
 7 int read()
 8 {
 9     char ch=getchar();int x=0;
10     while(ch>‘9‘||ch<‘0‘) ch=getchar();
11     while(ch>=‘0‘&&ch<=‘9‘) x=x*10+ch-‘0‘,ch=getchar();
12     return x;
13 }
14 int n,m,num,L,R,cnt;
15 int head[M],id[M],deep[M],f[M],fa[M],in[M];
16 int res1[M],res2[M],res3[M];
17 bool vis[M];
18 long long ans;
19 queue<int>Q;
20 struct point{int to,next;}e[M<<1];
21 void add(int from,int to)
22 {
23     e[++num].next=head[from];
24     e[num].to=to;
25     head[from]=num;
26 }
27 void dfs(int x,int fa)
28 {
29     for(int i=head[x];i;i=e[i].next)
30     {
31         int to=e[i].to;
32         if(to==fa) continue;
33         deep[to]=deep[x]+1;
34         dfs(to,x);
35     }
36 }
37 void mark(int x,int tar)
38 {
39     while(1)
40     {
41         vis[x]=true;int nxt=0;
42         if(x==tar) return;
43         for(int i=head[x];i;i=e[i].next)
44         {
45             int to=e[i].to;
46             if(deep[to]==deep[x]-1)
47             {
48                 nxt=to;
49                 break;
50             }
51         }
52         fa[x]=nxt;x=nxt;
53     }
54 }
55 int main()
56 {
57     n=read();
58     for(int a,b,i=1;i<n;i++)
59     {
60         a=read();b=read();
61         add(a,b); add(b,a);
62         in[a]++,in[b]++;
63     }
64     deep[1]=1;
65     dfs(1,0);
66     for(int i=1;i<=n;i++) if(deep[i]>deep[L]) L=i;
67     deep[L]=1; dfs(L,0);
68     for(int i=1;i<=n;i++) f[i]=deep[i]-1,id[i]=L;
69     for(int i=1;i<=n;i++) if(deep[i]>deep[R]) R=i;
70     deep[R]=1; dfs(R,0);
71     for(int i=1;i<=n;i++) if(deep[i]-1>f[i]) f[i]=deep[i]-1,id[i]=R;
72     deep[L]=1; dfs(L,0); mark(R,L);
73     for(int i=1;i<=n;i++) if(in[i]==1&&!vis[i]) Q.push(i);
74     while(!Q.empty())
75     {
76         int x=Q.front();Q.pop();
77         res1[++cnt]=x;res2[cnt]=id[x];res3[cnt]=x;
78         ans+=f[x];
79         for(int i=head[x];i;i=e[i].next)
80         {
81             int to=e[i].to;
82             in[to]--;
83             if(!vis[to]&&in[to]==1) Q.push(to);
84         }
85     }
86     for(int i=R;i!=L;i=fa[i])
87     {
88         res1[++cnt]=i;res2[cnt]=L;res3[cnt]=i;
89         ans+=deep[i]-deep[L];
90     }
91     printf("%lld\n",ans);
92     for(int i=1;i<=cnt;i++) printf("%d %d %d\n",res1[i],res2[i],res3[i]);
93     return 0;
94 }