Collect Coins in a Tree — C++

// Time:  O(n)
// Space: O(n)

// tree, bfs
class Solution {
public:
    int collectTheCoins(vector<int>& coins, vector<vector<int>>& edges) {
        static const int DISTANCE = 2;

        vector<unordered_set<int>> adj(size(coins));
        for (const auto& e : edges) {
            adj[e[0]].emplace(e[1]);
            adj[e[1]].emplace(e[0]);
        }
        int n = size(coins);
        for (int i = 0; i < size(coins); ++i) {
            int u = i;
            while (size(adj[u]) == 1 && !coins[u]) {
                const int v = *cbegin(adj[u]);
                adj[u].erase(v);
                adj[v].erase(u);
                --n;
                u = v;
            }
        }
        vector<int> q;
        for (int u = 0; u < size(coins); ++u) {
            if (size(adj[u]) == 1) {
                q.emplace_back(u);
            }
        }
        for (int _ = 0; _ < DISTANCE; ++_) {
            vector<int> new_q;
            for (const auto& u : q) {
                if (empty(adj[u])) {
                    assert(n == 1);
                    break;
                }
                const int v = *cbegin(adj[u]);
                adj[u].erase(v);
                adj[v].erase(u);
                --n;
                if (size(adj[v]) == 1) {
                    new_q.emplace_back(v);
                }
            }
            q = move(new_q);
        }
        return (n - 1) * 2;
    }
};