LeetCode 199. Binary Tree Right Side View

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<Integer> rightSideView(TreeNode root) {    List<Integer> list = new ArrayList<Integer>();        if (root != null) {            Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);        while (!queue.isEmpty()) {            Queue<TreeNode> tmp = new LinkedList<TreeNode>();        list.add(queue.peek().val);        while (!queue.isEmpty()) {        TreeNode tn = queue.poll();        if (tn.right != null) tmp.offer(tn.right);        if (tn.left != null) tmp.offer(tn.left);        }        queue = tmp;        }        }        return list;    }}