# 二叉树的层序遍历 II

<p>给定一个二叉树，返回其节点值自底向上的层序遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）</p>

<p>例如：<br />
给定二叉树 <code>[3,9,20,null,null,15,7]</code>,</p>

<pre>
    3
   / \
  9  20
    /  \
   15   7
</pre>

<p>返回其自底向上的层序遍历为：</p>

<pre>
[
  [15,7],
  [9,20],
  [3]
]
</pre>


## template

```python

class Solution(object):
    res = dict()

    def forwardSearch(self, root, depth):
        if root != None:
            if depth not in self.res.keys():
                self.res[depth] = []
            self.res[depth].append(root.val)
            self.forwardSearch(root.left, depth + 1)
            self.forwardSearch(root.right, depth + 1)

    def levelOrderBottom(self, root):
        self.forwardSearch(root, 1)
        result = []
        self.dic = dict()
        for i in self.res.keys():
            result.append(self.res[i])
        return result[::-1]
```

## 答案

```python

```

## 选项

### A

```python

```

### B

```python

```

### C

```python

```