# LeetCode – Minimum Height Trees (Java)

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.

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]

**Java Solution**

public List<Integer> findMinHeightTrees(int n, int[][] edges) { List<Integer> result = new ArrayList<Integer>(); if(n==0){ return result; } if(n==1){ result.add(0); return result; } ArrayList<HashSet<Integer>> graph = new ArrayList<HashSet<Integer>>(); for(int i=0; i<n; i++){ graph.add(new HashSet<Integer>()); } for(int[] edge: edges){ graph.get(edge[0]).add(edge[1]); graph.get(edge[1]).add(edge[0]); } LinkedList<Integer> leaves = new LinkedList<Integer>(); for(int i=0; i<n; i++){ if(graph.get(i).size()==1){ leaves.offer(i); } } if(leaves.size()==0){ return result; } while(n>2){ n = n-leaves.size(); LinkedList<Integer> newLeaves = new LinkedList<Integer>(); for(int l: leaves){ int neighbor = graph.get(l).iterator().next(); graph.get(neighbor).remove(l); if(graph.get(neighbor).size()==1){ newLeaves.add(neighbor); } } leaves = newLeaves; } return leaves; } |

<pre><code> String foo = "bar"; </code></pre>