leetcode Minimum Height Trees

寻找最小高度树
本文介绍了一种算法,用于从具有树特性的无向图中找到所有最小高度树(MHT)的根节点。通过逐步移除度数为1的叶节点直至剩余1-2个节点来确定MHT。

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 = 4edges = [[1, 0], [1, 2], [1, 3]]

        0
        |
        1
       / \
      2   3

return [1]

Example 2:

Given n = 6edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]

     0  1  2
      \ | /
        3
        |
        4
        |
        5

return [3, 4]

Show Hint 

    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.

    Credits:

    Special thanks to @dietpepsi for adding this problem and creating all test cases.

    提示说一棵树中最多有几颗MHT,答案是2,这个是可以证明的,但是博主并不会- -依照这个思路我们可以每次删去度数为1的叶结点,直到树中只剩1-2个结点即为所求,代码参考的https://discuss.leetcode.com/topic/48519/java-bfs-like-ac-solution-using-user-defined-graph-node-class,我觉得比我自己写的要清晰:

    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
            Map<Integer, Node> graph = new HashMap<>();
            for (int i = 0; i < n; i++) {
                graph.put(i, new Node(i));
            }
    
            for (int[] edge : edges) {
                graph.get(edge[0]).neighbors.add(graph.get(edge[1]));
                graph.get(edge[1]).neighbors.add(graph.get(edge[0]));
                graph.get(edge[0]).degree++;
                graph.get(edge[1]).degree++;
            }
    
            Queue<Node> queue = new LinkedList<>();
            for (int index : graph.keySet()) {
                if (graph.get(index).degree == 1) {
                    queue.offer(graph.get(index));
                }
            }
    
            while (n > 2) {
                int size = queue.size();
                for (int i = 0; i < size; i++) {
                    Node leaf = queue.poll();
                    Node neighbor = leaf.neighbors.iterator().next();
                    neighbor.neighbors.remove(leaf);//remove leaf from its neighbor's adj list
                    graph.remove(leaf.label);//remove leaf self
                    n--;
                    if (--neighbor.degree == 1) {
                        queue.offer(neighbor);
                    }
                }
            }
    
            return new ArrayList<>(graph.keySet());
    
        }
    }
    
    class Node {
        int label;
        int degree;
        Set<Node> neighbors;
        public Node(int index) {
            label = index;
            neighbors = new HashSet<>();
        }

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