<p>A tree is an undirected graph in which any two vertices are connected by&nbsp;<i>exactly</i>&nbsp;one path. In other words, any connected graph without simple cycles is a tree.</p>

<p>Given a tree of <code>n</code> nodes&nbsp;labelled from <code>0</code> to <code>n - 1</code>, and an array of&nbsp;<code>n - 1</code>&nbsp;<code>edges</code> where <code>edges[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> indicates that there is an undirected edge between the two nodes&nbsp;<code>a<sub>i</sub></code> and&nbsp;<code>b<sub>i</sub></code> in the tree,&nbsp;you can choose any node of the tree as the root. When you select a node <code>x</code> as the root, the result tree has height <code>h</code>. Among all possible rooted trees, those with minimum height (i.e. <code>min(h)</code>)&nbsp; are called <strong>minimum height trees</strong> (MHTs).</p>

<p>Return <em>a list of all <strong>MHTs&#39;</strong> root labels</em>.&nbsp;You can return the answer in <strong>any order</strong>.</p>

<p>The <strong>height</strong> of a rooted tree is the number of edges on the longest downward path between the root and a leaf.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/01/e1.jpg" style="width: 800px; height: 213px;" />
<pre>
<strong>Input:</strong> n = 4, edges = [[1,0],[1,2],[1,3]]
<strong>Output:</strong> [1]
<strong>Explanation:</strong> As shown, the height of the tree is 1 when the root is the node with label 1 which is the only MHT.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/09/01/e2.jpg" style="width: 800px; height: 321px;" />
<pre>
<strong>Input:</strong> n = 6, edges = [[3,0],[3,1],[3,2],[3,4],[5,4]]
<strong>Output:</strong> [3,4]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>edges.length == n - 1</code></li>
	<li><code>0 &lt;= a<sub>i</sub>, b<sub>i</sub> &lt; n</code></li>
	<li><code>a<sub>i</sub> != b<sub>i</sub></code></li>
	<li>All the pairs <code>(a<sub>i</sub>, b<sub>i</sub>)</code> are distinct.</li>
	<li>The given input is <strong>guaranteed</strong> to be a tree and there will be <strong>no repeated</strong> edges.</li>
</ul>

class Solution {
public:
vector<int> findMinHeightTrees(int n, vector<vector<int>>& edges) {
unordered_map<int,vector<int>> adj;

vector<int> degree(n,0);
if(edges.size() == 1)
{
    int u = edges[0][0];
    int v = edges[0][1];
    vector<int> ans;
    ans.push_back(u);
    ans.push_back(v);
    return ans; 
}
for(int i=0; i<edges.size(); i++)
{
    int u = edges[i][0];
    int v = edges[i][1];
    degree[u]++;
    degree[v]++;

    adj[u].push_back(v);
    adj[v].push_back(u);
}


unordered_map<int,bool> visited;
int ctr = n;
queue<int> q;
for(int i=0; i<n; i++)
{
    if(degree[i] == 1){
    q.push(i);
    visited[i]=true;
    ctr--;
    }
}

vector<int>ans; 
while(!q.empty())
{
    int size = q.size();
    vector<int> frr;
    for(int i=0; i <size; i++)
    {
        int fr = q.front();
        q.pop();

        for(auto it : adj[fr])
        {
            if(!visited[it])
            {
                degree[it]--;
                if(degree[it] == 1)
                {
                    frr.push_back(it);
                    ctr--;
                }
            }
        }
    }

    if(ctr == 0)
    {
        for(int i = 0; i<frr.size(); i++)
        ans.push_back(frr[i]);
        break;
    }
    
    for(int i=0; i<frr.size(); i++)
    {
        visited[frr[i]] = true;
        q.push(frr[i]);
    }
}

if(ans.size() == 0)
ans.push_back(0);

return ans;


        
        
        
    }
};


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