有点感动，这题在AS里写好之后复制过去一次AC了。
这种BFS的套路跟这题的第一个版本一样，用queue，维护curNum和nextNum；我跟code ganker学会了这个套路，按照覃超的标准来说这代码是有点长的，但是现在已经形成记忆了，就不去修改了。思路还蛮清晰的。

    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) return res;

        LinkedList<TreeNode> queue = new LinkedList<>();
        List<Integer> cell = new ArrayList<>();
        queue.add(root);
        int curNum = 1;
        int nextNum = 0;
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            cell.add(node.val);
            curNum--;

            if (node.left != null) {
                queue.offer(node.left);
                nextNum++;
            }

            if (node.right != null) {
                queue.offer(node.right);
                nextNum++;
            }

            if (curNum == 0) {
                res.add(0, cell);
                curNum = nextNum;
                nextNum = 0;
                cell = new ArrayList<>();
            }
        }
        return res;
    }

另

贴一下leetcode里面的人的简洁写法：https://discuss.leetcode.com/topic/7651/my-dfs-and-bfs-java-solution/2

public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
        
        if(root == null) return wrapList;
        
        queue.offer(root);
        while(!queue.isEmpty()){
            int levelNum = queue.size();
            List<Integer> subList = new LinkedList<Integer>();
            for(int i=0; i<levelNum; i++) {
                if(queue.peek().left != null) queue.offer(queue.peek().left);
                if(queue.peek().right != null) queue.offer(queue.peek().right);
                subList.add(queue.poll().val);
            }
            wrapList.add(0, subList);
        }
        return wrapList;
    }
}

递归（我没看）:

public class Solution {
        public List<List<Integer>> levelOrderBottom(TreeNode root) {
            List<List<Integer>> wrapList = new LinkedList<List<Integer>>();
            levelMaker(wrapList, root, 0);
            return wrapList;
        }
        
        public void levelMaker(List<List<Integer>> list, TreeNode root, int level) {
            if(root == null) return;
            if(level >= list.size()) {
                list.add(0, new LinkedList<Integer>());
            }
            levelMaker(list, root.left, level+1);
            levelMaker(list, root.right, level+1);
            list.get(list.size()-level-1).add(root.val);
        }
    }