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]
]
 

LeetCode：連結

# 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]]
        """
        if root == None:
            return []
        curStack = [root]
        result = []
        while curStack:
            '''列印當前行'''
            res = []
            '''儲存下一行'''
            nextStack = []
            for i in curStack:
                res.append(i.val)
                if i.left:
                    nextStack.append(i.left)
                if i.right:
                    nextStack.append(i.right)
            result.insert(0, res)
            '''將儲存的下一行直接賦值給新的要遍歷的內容
            也可以用pop 然後直接新增到curStack中'''
            curStack = nextStack
        return result