#include <bits/stdc++.h>
using namespace std;
const int mod=1e9+7;
int n,d;
vector<int> v[100001];
void read()
{
cin>>n>>d;
for(int i=1;i<n;i++)
{
int x,y;
cin>>x>>y;
v[x].push_back(y);
v[y].push_back(x);
}
}
int num;
int l[100001],w[100001];
void dfs1(int i,int p)
{
for(int j=0;j<v[i].size();j++)
{
int nb=v[i][j];
if(nb==p)continue;
dfs1(nb,i);
l[i]+=w[nb]^1;
}
if(l[i])w[i]=1;
}
int c[100001],sz[100001],up[100001];
void dfs2(int i,int p)
{
if(i!=1&&w[p]==0||w[p]==1&&l[p]==1&&!w[i])w[i]+=2;
//cout<<i<<" "<<w[i]<<endl;
if(w[i]==0)num++;
for(int j=0;j<v[i].size();j++)
{
int nb=v[i][j];
if(nb==p)continue;
dfs2(nb,i);
if(l[i]==1&&w[nb]!=1||l[i]==0)
sz[i]+=sz[nb];
}
if(w[i]%2==0)sz[i]++;
//cout<<i<<" "<<sz[i]<<endl;
}
void dfs3(int i,int p)
{
int add=up[p]+sz[p];
if(l[p]==1&&w[i]!=1||i!=1&&l[p]==0)
add-=sz[i];
up[i]=add;
c[i]=add+sz[i];
for(int j=0;j<v[i].size();j++)
{
int nb=v[i][j];
if(nb==p)continue;
dfs3(nb,i);
}
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
read();
dfs1(1,0);
dfs2(1,0);
dfs3(1,0);
if(!w[i])cout<<(sz[1]*num)%mod<<endl;
else cout<<(mod+n*n-(sz[1]*num)%mod)%mod<<endl;
return 0;
}
Compilation message
startrek.cpp: In function 'void dfs1(int, int)':
startrek.cpp:24:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
24 | for(int j=0;j<v[i].size();j++)
| ~^~~~~~~~~~~~
startrek.cpp: In function 'void dfs2(int, int)':
startrek.cpp:38:12: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
38 | if(i!=1&&w[p]==0||w[p]==1&&l[p]==1&&!w[i])w[i]+=2;
| ~~~~^~~~~~~~~
startrek.cpp:42:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
42 | for(int j=0;j<v[i].size();j++)
| ~^~~~~~~~~~~~
startrek.cpp:47:19: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
47 | if(l[i]==1&&w[nb]!=1||l[i]==0)
| ~~~~~~~^~~~~~~~~~
startrek.cpp: In function 'void dfs3(int, int)':
startrek.cpp:57:15: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
57 | if(l[p]==1&&w[i]!=1||i!=1&&l[p]==0)
| ~~~~~~~^~~~~~~~~
startrek.cpp:62:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
62 | for(int j=0;j<v[i].size();j++)
| ~^~~~~~~~~~~~
startrek.cpp: In function 'int main()':
startrek.cpp:80:8: error: 'i' was not declared in this scope
80 | if(!w[i])cout<<(sz[1]*num)%mod<<endl;
| ^
