POI 2014 HOTELS (树形DP)

 1 #include <bits/stdc++.h>
 2 
 3 using namespace std;
 4 
 5 #define rep(i, a, b)              for(int i(a); i <= (b); ++i)
 6 #define for_edge(i, x)            for(int i = H[x]; i; i = X[i])
 7 
 8 int n, x, y, et = 0;
 9 int E[100010], X[100010], H[100010], f[5010], g[5010], tmp[5010];
10 long long ans;
11 
12 inline void addedge(int a, int b){
13     E[++et] = b, X[et] = H[a], H[a] = et;
14     E[++et] = a, X[et] = H[b], H[b] = et;
15 }
16 
17 void dfs(int x, int fa, int dep){
18     ++tmp[dep];
19     for_edge(i, x){
20         int u = E[i];
21         if (u != fa) dfs(u, x, dep + 1);
22     }
23 }
24 
25 int main(){
26 
27     scanf("%d", &n);
28     rep(i, 1, n - 1){
29         scanf("%d%d", &x, &y);
30         addedge(x, y);
31     }
32 
33     rep(u, 1, n){
34         memset(f, 0, sizeof f);
35         memset(g, 0, sizeof g);
36         for_edge(i, u){
37             memset(tmp, 0, sizeof tmp);
38             x = E[i];
39             dfs(x, u, 1);
40             rep(j, 1, n){
41                 ans += (long long)g[j] * tmp[j];
42                 g[j] += f[j] * tmp[j];
43                 f[j] += tmp[j];
44             }
45         }
46     }
47 
48     printf("%lld\n", ans);    
49     return 0;
50 
51 }