310. Minimum Height Trees

For a undirected graph with tree characteristics, we can choose any node as the root. The result graph is then a rooted tree. Among all possible rooted trees, those with minimum height are called minimum height trees (MHTs). Given such a graph, write a function to find all the MHTs and return a list of their root labels.

Format
The graph contains n nodes which are labeled from 0 to n - 1. You will be given the number n and a list of undirected edges (each edge is a pair of labels).

You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

Example 1:

Given n = 4, edges = [[1, 0], [1, 2], [1, 3]]

        0
        |
        1
       / \
      2   3

return [1]

Example 2:

Given n = 6, edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]

     0  1  2
      \ | /
        3
        |
        4
        |
        5

return [3, 4]

借鉴的别人的代码：

 public class Solution {
     public List<Integer> findMinHeightTrees(int n, int[][] edges) {

         List<Integer> leaf=new ArrayList<>();
         if(n<=1)
         {
             leaf.add(0);
             return leaf;
         }

         Map<Integer,List<Integer>> graph=new HashMap<>();
         for(int i=0;i<n;i++)
         graph.put(i,new ArrayList());

         int[] neighbors=new int[n];
         for(int[] edge:edges)
         {
             neighbors[edge[0]]++;
             neighbors[edge[1]]++;
             graph.get(edge[0]).add(edge[1]);
             graph.get(edge[1]).add(edge[0]);
         }

         for(int i=0;i<n;i++)
         {
             if(graph.get(i).size()==1)
             leaf.add(i);
         }

         while(n>2)
         {
             List<Integer> newleaf=new ArrayList<>();
             for(int l:leaf)
             {
                 n--;
                 for(int nb:graph.get(l))
                 {
                     if(--neighbors[nb]==1)
                     newleaf.add(nb);
                 }

             }
             leaf=newleaf;
         }
         return leaf;
     }
 }