---
link: https://leetcode.com/problems/binary-tree-level-order-traversal-ii/
tags: tree, bt
---

# 107. Binary Tree Level Order Traversal II

## Question
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).

For example:
Given binary tree `[3,9,20,null,null,15,7]`,

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

## Solution: Python

from collections import deque

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        num_nodes = 1
        num_nodes_next_level = 0
        
        queue = deque()
        queue.append(root)
        result = deque()
        
        while queue:
            level = []
            
            for i in range(num_nodes):
                node = queue.popleft()
                
                if node is not None:
                    level.append(node.val)
                    
                    queue.append(node.left)
                    queue.append(node.right)
                    num_nodes_next_level += 2
            
            num_nodes = num_nodes_next_level
            num_nodes_next_level = 0
            
            if level:
                result.appendleft(level)
        
        return result

## Solution: Java