/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
 
        List<Integer> list = new ArrayList<>();
        if(root==null){
            return list;
        }
 
        LinkedList<TreeNode> queue = new LinkedList<>();        
        queue.add(root);
        while(!queue.isEmpty()){    //按层遍历二叉树
            int n = queue.size();  
            TreeNode node = null;          
            for(int i=0; i<n; i++){
                node = queue.poll();
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
            if(node!=null){ //把每一层末尾的节点值放到list中
                list.add(node.val);
            }
        }
        return list;
    }
}