leetcode515_树的每行的最大值

public List<Integer> largestValues(TreeNode root) {
    List<Integer> list = new ArrayList();
    if(root == null) return list;
    Deque<TreeNode> q = new LinkedList();
    q.offer(root);
    while(!q.isEmpty()) {
        int size = q.size();
        int ans = Integer.MIN_VALUE;
        for(int i = 0; i < size; i++) {
            TreeNode node = q.poll();
            if(node.left != null) q.offer(node.left);
            if(node.right != null) q.offer(node.right);
            ans = Math.max(ans, node.val);
        }
        list.add(ans);
    }
    return list;
}