Find Largest Value in Each Tree Row

You need to find the largest value in each row of a binary tree.

Example:

Input: 

          1
         / \
        3   2
       / \   \  
      5   3   9 

Output: [1, 3, 9]

class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null)
            return ans;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (q.size() != 0) {
            int size = q.size();
            int max = Integer.MIN_VALUE;
            while (size-- > 0) {
                TreeNode temp = q.poll();
                max = Math.max(max, temp.val);
                if (temp.left != null)
                    q.add(temp.left);
                if (temp.right != null)
                    q.add(temp.right);
            }
            ans.add(max);
        }
        return ans;
    }
}