bzoj 3522: [Poi2014]Hotel

 1 #include <bits/stdc++.h>
 2 #define LL long long
 3 #define lowbit(x) x&(-x)
 4 #define inf 1e15
 5 using namespace std;
 6 inline int ra()
 7 {
 8     int x=0,f=1; char ch=getchar();
 9     while (ch<‘0‘ || ch>‘9‘) {if (ch==‘-‘) f=-1; ch=getchar();}
10     while (ch>=‘0‘ && ch<=‘9‘) {x=x*10+ch-‘0‘; ch=getchar();}
11     return x*f;
12 }
13 struct edge{int next,to;}e[10005];
14 int tot[5005],mx,n,head[5005],cnt,s1[5005],s2[5005];
15 LL ans;
16 void insert(int x, int y){e[++cnt].next=head[x]; e[cnt].to=y; head[x]=cnt;}
17 void dfs(int x, int fa, int deep)
18 {
19     tot[deep]++;
20     mx=max(mx,deep);
21     for (int i=head[x];i;i=e[i].next)
22     {
23         if (e[i].to==fa) continue;
24         dfs(e[i].to,x,deep+1);
25     }
26 }
27 int main(int argc, char const *argv[])
28 {
29     n=ra();
30     for (int i=1; i<n; i++)
31     {
32         int x=ra(),y=ra();
33         insert(x,y); insert(y,x);
34     }
35     for (int x=1; x<=n; x++)
36     {
37         memset(s1,0,sizeof(s1));
38         memset(s2,0,sizeof(s2));
39         for (int i=head[x];i;i=e[i].next)
40         {
41             dfs(e[i].to,x,1);
42             for (int j=1; j<=mx; j++)
43             {
44                 ans+=s2[j]*tot[j];
45                 s2[j]+=s1[j]*tot[j];
46                 s1[j]+=tot[j];
47             }
48             for (int j=1; j<=mx; j++) tot[j]=0;
49         }
50     }
51     cout<<ans;
52     return 0;
53 }