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]

Hint:

1. How many MHTs can a graph have at most?

Note:

(1) According to the definition of tree on Wikipedia: “a tree is an undirected graph in which any two vertices are connected by exactly one path. In other words, any connected graph without simple cycles is a tree.”

(2) The height of a rooted tree is the number of edges on the longest downward path between the root and a leaf.

Solution:

Keep removing the most outside node util there is only one node left or two nodes left, what left is the answer.

Code:

public class Solution {    public List<Integer> findMinHeightTrees(int n, int[][] edges) {        int[] indegree = new int[n];        HashMap<Integer,HashSet<Integer>> hm = new HashMap();        for(int[] e : edges){            indegree[e[0]] += 1;            indegree[e[1]] += 1;            hm.computeIfAbsent(e[0], k -> new HashSet<Integer>()).add(e[1]);            hm.computeIfAbsent(e[1], k -> new HashSet<Integer>()).add(e[0]);        }                ArrayList<Integer> next = new ArrayList<Integer>();        int total = n;        for(int i = 0; i < n; i++){            if(indegree[i] <= 1){ // use <=  here because of the special case [1]                next.add(i);            }        }                while(total > 2){            ArrayList<Integer> temp = new ArrayList<>();            for(int i : next){                total -= 1;                for(int j : hm.get(i)){                    indegree[j] -= 1;                    if(indegree[j] == 1){                        temp.add(j);                    }                }            }            next = temp;        }        return next;            }}