#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
using namespace std;
const ll MOD = 1000000007;
ll N, D, dp[100005][2], pre[100005];
vector<int> E[100005];
void dfs(int k)
{
for (int i : E[k])
if (i != pre[k])
{
pre[i] = k;
dfs(i);
if (dp[i][0] == 0 && dp[k][0] == 0)
dp[k][0] = i;
else if (dp[i][0] == 0)
dp[k][1] = i;
}
}
void reroot(int k)
{
for (int i : E[k])
if (i != pre[k])
{
if ((dp[k][0] == 0) || (dp[k][0] == i && dp[k][1] == 0))
{
if (dp[i][0] == 0)
dp[i][0] = k;
else
dp[i][1] = k;
}
reroot(i);
}
}
ll dfs2(int k, bool first)
{
if (first)
if (dp[k][1] != 0)
return 0;
else
return dfs2(dp[k][0], false);
else
{
int ans = 1;
for (int i : E[k])
if (i != pre[k])
ans += dfs2(i, true);
return ans;
}
}
signed main()
{
cin >> N >> D;
if (N == 2)
{
ll p = D % (MOD - 1), b = 4;
while (p)
{
if (p & 1)
ans = (ans * b) % MOD;
p >>= 1;
b = b * b % MOD;
}
cout << ans << '\n';
return 0;
}
assert(D == 1);
for (int i = 1; i < N; i++)
{
int u, v;
cin >> u >> v;
E[u].emplace_back(v);
E[v].emplace_back(u);
}
dfs(1);
ll cnt = dfs2(1, dp[1][0] != 0);
reroot(1);
ll lose = 0;
for (int i = 1; i <= N; i++)
if (dp[i][0] == 0)
{
lose++;
//cerr << i << '\n';
}
if (dp[1][0])
cout << (N * N - cnt * lose) % MOD << '\n';
else
cout << (cnt * lose) % MOD << '\n';
}
Compilation message
startrek.cpp: In function 'int main()':
startrek.cpp:70:17: error: 'ans' was not declared in this scope; did you mean 'abs'?
70 | ans = (ans * b) % MOD;
| ^~~
| abs
startrek.cpp:74:17: error: 'ans' was not declared in this scope; did you mean 'abs'?
74 | cout << ans << '\n';
| ^~~
| abs
