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;
}