Find Largest Value in Each Tree Row

Given the root of a binary tree, return an array of the largest value in each row of the tree (0-indexed).

Example 1:

Input: root = [1,3,2,5,3,null,9]
Output: [1,3,9]

Example 2:

Input: root = [1,2,3]
Output: [1,3]

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,null,2]
Output: [1,2]

Example 5:

Input: root = []
Output: []

Constraints:

The number of nodes in the tree will be in the range [0, 104].
-231 <= Node.val <= 231 - 1

Solution:

/**
 * 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<Integer> largestValues(TreeNode root) {
        if (root == null) return new ArrayList();
        List<Integer> res = new ArrayList();
        Deque<TreeNode> queue = new ArrayDeque();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            int max = Integer.MIN_VALUE;
            for (int i = 0; i < size; i ++) {
                TreeNode curr = queue.poll();
                max = Math.max(max, curr.val);
                if (curr.left != null) queue.offer(curr.left);
                if (curr.right != null) queue.offer(curr.right);
            }
            res.add(max);
        }
        return res;
    }
}