breadth_first_search_algorithm

#include <iostream>
#include <vector>
#include <deque>

using namespace std;

int breadth_first_search_algorithm(vector<vector<int>>& g, vector<int>& seen, int start) {
  deque q{{start}};
  seen[start] = 1; // mark as seen

  int dist = 0;
  while (not q.empty()) {
    size_t s = q.size();
    while (s--) {
      const int curr = q.front(); q.pop_front();
      for (const int nei : g[curr]) {
        if (seen[nei]) continue;
        q.emplace_back(nei);
        seen[nei] = 1;
      }
    }
    ++dist;
  }

  return dist - 1;
}

int main(const int argc, const char** argv) {
  int n, m, u, v;
  cin >> n >> m;
  vector<vector<int>> g(n + 1);
  vector<int> seen(n + 1, 0);
  for (int i = 0; i < n - 1; ++i) {
    cin >> u >> v;
    g[u].emplace_back(v);
    g[v].emplace_back(u);
  }

  int distance = breadth_first_search_algorithm(g, seen, m);
  cout << distance;
  return 0;
}