Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

思路：宽度搜索每一层，把每一层最后一个点放入结果集中。

public List<Integer> rightSideView(TreeNode root) {
    List<Integer> res = new ArrayList<>();
    if (root == null) {
        return res;
    }

    Queue<TreeNode> q = new LinkedList<>();
    q.offer(root);
    while (!q.isEmpty()) {
        int len = q.size();
        for (int i = 0; i < len - 1; i++) {
            TreeNode cur = q.poll();
            if (i == len - 1) {
                res.add(cur.val);
            }
            if (cur.left != null) {
                q.offer(cur.left);
            }
            if (cur.right != null) {
                q.offer(cur.right);
            }
        }
    }

    return res;
}