Revise paste Untitled

#include <iostream>
#include <vector>
#include <queue>
using namespace std;
const int maxn=500000;
vector<int> graph[maxn];
pair<int, int> bfs(int S) {
    queue<int> q;
    q.push(S);
    q.push(0);
    vector<bool> visited(maxn, false);
    visited[S] = true;
    
    int max_dist = 0;
    int furthest_node = -1;
    while(!q.empty()) {
        int node = q.front();
        q.pop();
        int dist = q.front();
        q.pop();
        
        if(max_dist < dist) {
            max_dist = dist;
            furthest_node = node;
        }
        for(int i = 0; i < (int) graph[node].size(); i++) {
            int neighbour = graph[node][i];
            if(!visited[neighbour]) {
                visited[neighbour] = true;
                q.push(neighbour);
                q.push(dist + 1);
            }
        }
        
    }
    return make_pair(furthest_node, max_dist);
}
int main() {
    int n;
    cin >> n;
    if(n == 1) {
        cout << 0 << endl;
        return 0;
    }
    for(int i = 1; i < n; i++) {
        int a, b;
        cin >> a >> b;
        a--; b--;
        graph[a].push_back(b);
        graph[b].push_back(a);
    }
    int farthest_node_from_node_0 = bfs(0).first;
    int res = bfs(farthest_node_from_node_0).second;
    
    cout << res << endl;
    return 0;
}