Description：
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.

Example:

Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

题意：给定一颗二叉树，从二叉树的右侧方向看去，输出所能看到的每一层的节点；

解法：这道题其实和层次遍历是相同的做法，只不过我们每次只需要找到每一层的最后一个节点；因此，我们用一个变量记录添加的每层的节点，这样便可以找到每一层的最后一个节点；同样的，利用队列来实现层次遍历；

Java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.addLast(root);
        int len = 1;
        while (!queue.isEmpty()) {
            int temp = len;
            len = 0;
            while (temp-- > 0) {
                TreeNode node = queue.poll();
                if (temp == 0) {
                    result.add(node.val);
                }
                if (node.left != null) {
                    queue.addLast(node.left);
                    len++;
                }
                if (node.right != null) {
                    queue.addLast(node.right);
                    len++;
                }
            }
        }
        
        return result;
    }
}