# LC310. Minimum Height Trees

## Problem Description​

https://leetcode.com/problems/minimum-height-trees/

For an 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 :

``Input: n = 4, edges = [[1, 0], [1, 2], [1, 3]]        0        |        1       / \      2   3 Output: ``

Example 2 :

``Input: n = 6, edges = [[0, 3], [1, 3], [2, 3], [4, 3], [5, 4]]     0  1  2      \ | /        3        |        4        |        5 Output: [3, 4]``

## Solution​

``class Solution {    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {        List<List<Integer>> toRet = new ArrayList<>();        if (root == null) return toRet;        Queue<TreeNode> queue = new LinkedList<>();        queue.add(root);        int count;        boolean leftToRight = true;        while(!queue.isEmpty()){            count = queue.size();            ArrayList<Integer> level = new ArrayList<>();            for (int i = 0; i < count; i ++){                TreeNode node = queue.poll();                if(leftToRight) {                    level.add(node.val);                } else {                    level.add(0, node.val);                }                if(node.left != null){                    queue.add(node.left);                }                if(node.right != null){                    queue.add(node.right);                }            }            leftToRight = !leftToRight;            toRet.add(level);        }        return toRet;    }}``