给定一个二叉树,返回其节点值自底向上的层次遍历。 (即按从叶子节点所在层到根节点所在的层,逐层从左向右遍历)
例如:
给定二叉树 [3,9,20,null,null,15,7]
,
3 / \ 9 20 / \ 15 7
返回其自底向上的层次遍历为:
[ [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 = None class Solution(object): def levelOrderBottom(self, root): """ :type root: TreeNode :rtype: List[List[int]] """ if root == None: return [] temp=[] res=[] stack=[] n=1 stack.append(root) while stack: #stack用来遍历树的所有元素,temp存储每层的元素,n判断该层元素是否处理完 node=stack.pop(0) n-=1 temp+=[node.val] if node.left!=None: stack.append(node.left) if node.right!=None: stack.append(node.right) if n == 0: res.insert(0,temp) temp=[] n=len(stack) return res