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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3.  
  4. const long long MaxN = 1e6;
  5.  
  6. long long n;
  7. vector<long long> a[MaxN+5];
  8. bool visited[MaxN+5];
  9. long long depth1[MaxN+5],depth2[MaxN+5];
  10.  
  11. void input()
  12. {
  13.     cin >> n;
  14.  
  15.     for(long long i=1;i<=n-1;i++)
  16.     {
  17.         long long u,v;
  18.         cin >> u >> v;
  19.         a[u].push_back(v);
  20.         a[v].push_back(u);
  21.     }
  22. }
  23.  
  24. void dfs(long long i,vector<long long> a[],bool visited[],long long depth[])
  25. {
  26.     stack<long long> st;
  27.     st.push(i);
  28.     visited[i]=true;
  29.  
  30.     while(!st.empty())
  31.     {
  32.         long long u=st.top();
  33.         st.pop();
  34.  
  35.         for(long long v:a[u])
  36.         {
  37.             if(!visited[v])
  38.             {
  39.                 visited[v]=true;
  40.                 depth[v]=depth[u]+1;
  41.                 st.push(v);
  42.             }
  43.         }
  44.     }
  45. }
  46.  
  47. void solve()
  48. {
  49.     memset(visited,false,sizeof(visited));
  50.     memset(depth1,0,sizeof(depth1));
  51.  
  52.     dfs(1,a,visited,depth1);
  53.  
  54.     long long p1=1;
  55.  
  56.     for(long long i=2;i<=n;i++)
  57.     {
  58.         if(depth1[i]>depth1[p1])
  59.         {
  60.             p1=i;
  61.         }
  62.     }
  63.  
  64.     memset(visited,false,sizeof(visited));
  65.     memset(depth2,0,sizeof(depth2));
  66.  
  67.     dfs(p1,a,visited,depth2);
  68.  
  69.     long long p2=p1;
  70.  
  71.     for(long long i=2;i<=n;i++)
  72.     {
  73.         if(depth2[i]>depth2[p2])
  74.         {
  75.             p2=i;
  76.         }
  77.     }
  78.  
  79.     cout << depth2[p2];
  80. }
  81.  
  82. int main()
  83. {
  84.     ios_base::sync_with_stdio(0);
  85.     cin.tie(0);
  86.  
  87.     input();
  88.     solve();
  89. }
Success #stdin #stdout 0.01s 43648KB
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https://ideone.com/6n4aya
language:
C++14 (gcc 8.3)
created:
7 hours ago
visibility:
public

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