leetcode + minimum height tree+拨洋葱法，最后结果要么是一个或者两个

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class Solution {
public:
    vector<int> findMinHeightTrees(int n, vector<pair<int, int>>& edges) {
        if(n==1) return {0};
        vector<int> res;
        vector<unordered_set<int>> adj(n);
        queue<int> Q;
        for(int i=0; i<edges.size();i++){
            adj[edges[i].first].insert(edges[i].second);
            adj[edges[i].second].insert(edges[i].first);
        }
        for(int i=0; i<n; i++){ //叶子节点
            if(adj[i].size()==1) Q.push(i);
        }
        while (n>2) {
            int size = Q.size();
            n -= size;
            for(int i=0; i<size; i++){
                int t = Q.front(); Q.pop();
                for(auto a: adj[t]){
                    adj[a].erase(t);
                    if(adj[a].size()==1) Q.push(a);
                }
            }
        }
        while (!Q.empty()) {
            res.push_back(Q.front()); Q.pop();
        }
        return res;
    }
};