Description
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]
]
sln1
看到題目很直接就想到用廣度優先搜索,用遞歸實現。python代碼如下
# 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 []
return self.bfs([root])
def bfs(self, nodes):
if len(nodes) == 0:
return []
branch = []
values = []
for node in nodes:
values.append(node.val)
if node.left != None:
branch.append(node.left)
if node.right != None:
branch.append(node.right)
next_layer_values = self.bfs(branch)
next_layer_values.append(values)
return next_layer_values
每一次遞歸一個循環,分別記錄values和branch數組。values數組記錄當前層每個節點的val,branch數組記錄當前層的所有子節點。接下來調用bfs方法獲取到當前層以下的所有層的遍歷結果,再把values數組添加進去並返回。思路十分清晰,但是提交後看結果並不是十分理想,同樣是python實現,只超過了10%+的提交。
sln2
參考discussion中一個標題爲 C++ 4ms solution 的答案,用python簡答覆現了一次,提交結果盡然比70%+的結果快。python實現如下:
# 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 []
depth = self.depth(root)
return self.levelOrder([[] for _ in xrange(depth)], root, depth - 1)
def levelOrder(self, ans, node, depth):
if node is None:
return ans
ans[depth].append(node.val)
ans = self.levelOrder(ans, node.left, depth - 1)
ans = self.levelOrder(ans, node.right, depth - 1)
return ans
def depth(self, node):
if node is None:
return 0
return max(self.depth(node.left), self.depth(node.right)) + 1
一開始並不是很理解爲什麼這個實現會比我的第一種實現更快,思考一下之後發現,第一種實現方法裏面很顯然是有一個循環的。在這個循環中,我們遍歷了一遍父節點,並把子節點都壓進一個數組中,相當於也遍歷了一遍子節點。當我們把這個子節點數組丟進下一次遞歸時,又會再遍歷一次,所以實際上等於所有子節點都遍歷了兩次,所以速度上就慢了很多