/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        int size = 1;
        int i = 0;
        int max = Integer.MIN_VALUE;
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode current = queue.poll();
            max = Math.max(max, current.val);
            if (current.left != null) {
                queue.offer(current.left);
            }
            if (current.right != null) {
                queue.offer(current.right);
            }
            
            i++;
            if (size == i) {
                result.add(max);
                max = Integer.MIN_VALUE;
                size = queue.size();
                i = 0;
            }
        }
        
        return result;
    }
}