Diameter using single DFS in O(n).

diameter.cpp

#include <bits/stdc++.h>
using namespace std;

const int N = 2e5 + 1;
VI adj[N];

int mdep[N];
int diam = 0;

void dfs(int u, int p = -1, int d = 0, int pdep = 0) {
  mdep[u] = d;
  diam = max(diam, pdep + d);
  for (int v : adj[u]) {
    if (v == p) continue;
    dfs(v, u, d + 1, pdep);
    mdep[u] = max(mdep[u], mdep[v]);
    pdep = max(pdep, mdep[u] - 2 * d);
  }
}

int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  dfs(1);
  cout << diam << endl;

  return 0;
}