leetcode199_二叉树的右视图

public List rightSideView(TreeNode root) {
    List list = new ArrayList();
    if(root == null) return list;
    Deque q = new LinkedList();
    q.offer(root);
    while(!q.isEmpty()) {
        int size = q.size();
        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);
            if(i == size-1) list.add(node.val);
        }
    }
    return list;
}