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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: [1]

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;
    }
}