3
   / \
  9  20
    /  \
   15   7

[
  [15,7],
  [9,20],
  [3]
]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> levelOrder = new LinkedList<>();
        if(root == null){
            return levelOrder;
        }
        //创建队列
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        //将根节点加入到队列中
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> level = new ArrayList<>();
            //获取当前层的TreeNode的个数
            int size = queue.size();
            //遍历取出当前层的所有TreeNode，并将下一层的加入
            for(int i = 0;i < size;i++){
                TreeNode node = queue.poll();
                level.add(node.val);
                if(node.left!=null){
                    queue.offer(node.left);
                }
                 if(node.right!=null){
                    queue.offer(node.right);
                }
            }
            levelOrder.addFirst(level);
        }
        return levelOrder;
​
    }
}