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题目：
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]]
# Definition for a binary tree node.class TreeNode(object): def __init__(self, x): self.val = x self.left = None self.right = Noneclass Solution(object): def levelOrderBottom(self, root): """ :type root: TreeNode :rtype: List[List[int]] """ results = [] if root == None: return results curr_level = [root] while curr_level: curr_result = [] next_level = [] for i in curr_level: curr_result.append(i.val) if i.left: next_level.append(i.left) if i.right: next_level.append(i.right) results.append(curr_result) curr_level = next_level results.reverse() return resultsif __name__ == '__main__': l1_1 = TreeNode(1) l1_2 = TreeNode(2) l1_3 = TreeNode(3) l1_1.left = l1_2 l1_1.right = l1_3 l3 = Solution().levelOrderBottom(l1_1) print l3
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