Leetcode 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]

Note:

(1) According to the definition of tree on Wikipedia: “a tree is an undirected graph in which any two vertices are connected by exactlyone 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.

当n 等于 2 时， 直接返回 0， 1.

把子节点数从 n 中 减去，并找到 子节点 间接的点，然后依次减去子节点。重复这项操作直到 n 小于或等于 2.

public List<Integer> findMinHeightTrees(int n, int[][] edges) {        if (n == 1) {            List<Integer> res = new ArrayList<>();            res.add(0);            return res;        }        List<Set<Integer>> adj = new ArrayList<>();        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;    }