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;
}
}