python寫算法題：leetcode: 107. Binary Tree Level Order Traversal II

class Solution(object):
    def step(self, root, ret, level):
        if root==None: return
        if level>=len(ret):
            ret.insert(0,[])
        ret[len(ret)-level-1].append(root.val)
        print root.val, level
        self.step(root.left, ret, 1+level)
        self.step(root.right, ret, 1+level)
        
        
        
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        ret=[]
        self.step(root, ret, 0)
        return ret