310. Minimum Height Trees LeetCode

题意：给出一张图，选一个节点作为根使得这棵树最矮，求所有这样的节点。
题解：有点像拓扑排序的做法，就是从度为1的点一层一层往里面扩展，最后剩下的点就是树根。

class Solution {public:    vector<int> findMinHeightTrees(int n, vector<pair<int, int>>& edges) {        if(n == 1) return vector<int>{0};        vector<vector<int>> edge;        vector<int> vis,d,ans;        for(int i = 0; i <= n; i++)        {            edge.push_back(vector<int>{});            vis.push_back(0);            d.push_back(0);        }        for(int i = 0; i < edges.size(); i++)        {            int u = edges[i].first;            int v = edges[i].second;            edge[u].push_back(v);            edge[v].push_back(u);            d[u]++;            d[v]++;        }        queue<int> q;        while(!q.empty()) q.pop();        for(int i = 0; i < n; i++)            if(d[i] == 1) q.push(i),vis[i] = 1;        while(!q.empty())        {            int n = q.size();            ans.clear();            for(int k = 1; k <= n; k++)            {                int now = q.front();                q.pop();                ans.push_back(now);                for(int i = 0; i < edge[now].size(); i++)                {                    int v = edge[now][i];                    if(!vis[v])                    {                        d[v]--;                        if(d[v] == 1)                        {                            q.push(v);                            vis[v] = 1;                        }                    }                }            }        }        return ans;    }};