leetcode 310. 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]
Solution. The solution is easy. Delete the leaves in each round until there are 1 or 2 nodes. That nodes will be the result.
2 things are worth to notice:
1. Because tree nodes are marked by 0, 1, 2, 3, 4 … Consider constructing tree in ArrayList<HashSet<Integer>>, instead of HashMap<Integer, HashSet<Integer>>
2. Maintain a leave set, each time remove the leaves. Then update leave. Once the tree is built, this process takes O(V) time, which V is the number of nodes.
Check my code on github: link