Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
Example 1:
Input: root = [3,9,20,null,null,15,7] Output: [[3],[9,20],[15,7]]
Example 2:
Input: root = [1] Output: [[1]]
Example 3:
Input: root = [] Output: []
Constraints:
[0, 2000].-1000 <= Node.val <= 1000/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> levels = new ArrayList<>();
if (root == null)
return levels;
Queue<TreeNode> q = new LinkedList<>();
q.offer(root);
while (!q.isEmpty()) {
int size = q.size();
List level = new ArrayList<>();
while (size-- > 0) {
TreeNode tn = q.poll();
level.add(tn.val);
if (tn.left != null)
q.offer(tn.left);
if (tn.right != null)
q.offer(tn.right);
}
levels.add(level);
}
return levels;
}
}