Leetcode: 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:

How many MHTs can a graph have at most? Answer： 1 or 2

build graph first, also buil indegree array, then find leaf and remove them among their neighbors, level by level. Until left less 2 nodes

public class Solution {
public List<Integer> findMinHeightTrees(int n, int[][] edges) {
List<Integer> leaves = new ArrayList<Integer>();
if (edges==null || edges.length==0) {
leaves.add(n-1);
return leaves;
}
HashMap<Integer, ArrayList<Integer>> graph = new HashMap<Integer, ArrayList<Integer>>();
int[] indegree = new int
;

for (int i=0; i<n; i++) {
graph.put(i, new ArrayList<Integer>());
}

//build the graph
for (int[] edge : edges) {
graph.get(edge[0]).add(edge[1]);
graph.get(edge[1]).add(edge[0]);
indegree[edge[0]]++;
indegree[edge[1]]++;
}

//find the leaves
for (int i=0; i<n; i++) {
if (indegree[i] == 1) {
leaves.add(i);
}
}

//topological sort until n<=2
while (n > 2) {
List<Integer> newLeaf = new ArrayList<Integer>();
for (Integer leaf : leaves) {
List<Integer> neighbors = graph.get(leaf);
for (Integer neighbor : neighbors) {
indegree[neighbor]--;
graph.get(neighbor).remove(leaf);
if (indegree[neighbor] == 1)
newLeaf.add(neighbor);
}
//delete leaf from graph
indegree[leaf]--;
neighbors.clear();
n--;
}
leaves = newLeaf;
}

return leaves;
}
}