parents-levels-tree.cpp

vector <vector <int> > adj; // adjacency list vectors

void dfs(int i, int par){
  // set parent of i as the value passed in par
  parent[i] = par;
  
  if(par == -1)   // means i is root
    level[i] = 0;
  else            // otherwise
    level[i] = level[parent[i]]+1
    
  for(auto j: adj[i]){
    if(j == par)    // don't go to parent again! :/
      continue;
    dfs(j, i);      // go to children
  }
}

int main(){
  int n; // n = # nodes, we don't need # edges
  cin >> n;
  adj.resize(n+1);
  for(int i = 0, x, y; i < n-1; ++i){ // read n-1 edges
    cin >> x >> y;
    // push to adjacency list
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  // Let root be 1, so we pass the parent as -1 (see dfs function)
  dfs(1, -1);
}