##Binary Tree Level Order Traversal II
https://leetcode.com/problems/binary-tree-level-order-traversal-ii/
Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).
class Solution:
# @param root, a tree node
# @return a list of lists of integers
def levelOrderBottom(self, root):
if not root:
return[]
result = []
self.helper(root, 0, result)
result.reverse()
return result
def helper(self, root, level, result):
if not root:
return
if level+1 > len(result):
result.append([])
result[level].append(root.val)
self.helper(root.left, level+1, result)
self.helper(root.right, level+1, result)