Minimum Height Trees
Source
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:
How many MHTs can a graph have at most?
题解
Java
public List<Integer> findMinHeightTrees(int n, int[][] edges) {
if (n == 1) return Collections.singletonList(0);
List<Set<Integer>> adj = new ArrayList<>(n);
for (int i = 0; i < n; ++i) adj.add(new HashSet<>());
for (int[] edge : edges) {
adj.get(edge[0]).add(edge[1]);
adj.get(edge[1]).add(edge[0]);
}
List<Integer> leaves = new ArrayList<>();
for (int i = 0; i < n; ++i)
if (adj.get(i).size() == 1) leaves.add(i);
while (n > 2) {
n -= leaves.size();
List<Integer> newLeaves = new ArrayList<>();
for (int i : leaves) {
int j = adj.get(i).iterator().next();
adj.get(j).remove(i);
if (adj.get(j).size() == 1) newLeaves.add(j);
}
leaves = newLeaves;
}
return leaves;
}