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 3return [1]
Example 2:
Given n = 6, edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]
0 1 2 \ | / 3 | 4 | 5return [3, 4]
JAVA代码： public class Solution { public static List<Integer> findMinHeightTrees(int n, int[][] edges) { ArrayList<HashSet<Integer>> G=new ArrayList<HashSet<Integer>>(); for(int i=0;i<n;i++) G.add(new HashSet<Integer>()); for(int[] edg:edges){ G.get(edg[0]).add(edg[1]); G.get(edg[1]).add(edg[0]); } LinkedList<Integer> leaves=new LinkedList<Integer>(); if(n==1)leaves.add(0); for(int i=0;i<n;i++)if(G.get(i).size()==1)leaves.add(i); while(n>2){ n-=leaves.size(); LinkedList<Integer> newleaves=new LinkedList<Integer>(); for(int i:leaves){ int j=G.get(i).iterator().next(); G.get(j).remove(i); if(G.get(j).size()==1)newleaves.add(j); } leaves=newleaves; } return leaves; } } 时间复杂度 O(N)