107. Binary Tree Level Order Tra

其实和102. Binary Tree Level Order Traversal这题几乎一样的思路和解法。
不赘述，直接给代码。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
        result = []
        if root is None:
            return result
        
        queue = [root]
        while len(queue):
            length = len(queue)
            level = []
            for i in range(length):
                p = queue.pop(0)
                level.append(p.val)
                if p.left:
                    queue.append(p.left)
                if p.right:
                    queue.append(p.right)
            result.insert(0, level)
        return result