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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3.  
  4. #define int long long
  5.  
  6. const int N = 200005;
  7.  
  8. int n, k, ans;
  9. vector<int> adj[N];
  10.  
  11. map<int,int>* dfs(int u, int p) {
  12.  
  13.     auto *mp = new map<int,int>();
  14.     (*mp)[0] = 1;
  15.  
  16.     for (int v : adj[u]) {
  17.  
  18.         if (v == p) continue;
  19.  
  20.         auto *child = dfs(v, u);
  21.  
  22.         // luôn merge nhỏ vào lớn
  23.         if (mp->size() < child->size())
  24.             swap(mp, child);
  25.  
  26.         // đếm đáp án
  27.         for (auto [d, f] : *child) {
  28.  
  29.             int need = k - d - 1;
  30.  
  31.             if (mp->count(need))
  32.                 ans += 1LL * f * (*mp)[need];
  33.         }
  34.  
  35.         // merge
  36.         for (auto [d, f] : *child) {
  37.             (*mp)[d + 1] += f;
  38.         }
  39.     }
  40.  
  41.     return mp;
  42. }
  43.  
  44. signed main() {
  45.  
  46.     ios::sync_with_stdio(false);
  47.     cin.tie(nullptr);
  48.  
  49.     cin >> n >> k;
  50.  
  51.     for (int i = 1; i < n; i++) {
  52.         int u, v;
  53.         cin >> u >> v;
  54.  
  55.         adj[u].push_back(v);
  56.         adj[v].push_back(u);
  57.     }
  58.  
  59.     dfs(1, 0);
  60.  
  61.     cout << ans;
  62. }
Success #stdin #stdout 0.01s 8172KB
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https://ideone.com/nUMheZ
language:
C++14 (clang 8.0)
created:
8 hours ago
visibility:
public

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