hdu 4705 dfs统计更新节点信息

#pragma comment(linker, "/STACK:16777216")
#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <queue>
#include <vector>
using namespace std;

const int maxn = 110010;

long long  dp[maxn];
vector<int> G[maxn];
long long num[maxn];
long long N;
long long ans;

void dfs(int u,int fa){
for(int i=0;i<G[u].size();i++){
int v = G[u][i];
if(v == fa)  continue;
dfs(v,u);
num[u] += num[v];
}
long long numfa = N - num[u];
for(int i=0;i<G[u].size();i++){
int v = G[u][i];
if(v == fa){
long long  temp = num[u]-1;
dp[u] += numfa * temp;
continue;
}
long long  temp = N - num[v] - 1;
dp[u] += num[v] * temp;
}
ans += dp[u];
}

int main()
{
// freopen("E:\\acm\\input.txt","r",stdin);
while(cin>>N){
memset(dp,0,sizeof(dp));
for(int i=1;i<=N;i++)  num[i] = 1;
for(int i=1;i<=N;i++)  G[i].clear();
int a,b;
for(int i=1;i<=N-1;i++){
scanf("%d%d",&a,&b);
G[a].push_back(b);
G[b].push_back(a);
}
ans = 0;
dfs(1,-1);
long long  MAX;
MAX = N*(N-1)*(N-2);   //N写成int，比赛时wa的太痛苦了，还是队友看出来了。以后要注意了。

cout<<(MAX-ans*3)/6<<endl;
}
return 0;
}