LeetCode Problem 107-Binary Tree Level Order Traversal II

二叉树的层次遍历 II。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],

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    3
   / \
  9  20
    /  \
   15   7

返回其自底向上的层次遍历为:

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[
  [15,7],
  [9,20],
  [3]
]
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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

思路一

广度优先遍历,队列解决。时间复杂度

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from collections import deque

class Solution:
    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
        q, rs = deque(), []
        q.append(root)
        while q:
            level = len(q)
            tmp = []
            for _ in range(level):
                node = q.popleft()
                if node is None:
                    continue
                tmp.append(node.val)
                q.append(node.left)
                q.append(node.right)
            if tmp:
                rs.insert(0, tmp)
        return rs

思路二

广度优先遍历,递归解决。时间复杂度

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class Solution:
    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
        def backtrack(node, level=0):
            if not node:
                return
            if level >= len(rs):
                rs.insert(0, [])
            backtrack(node.left, level+1)
            backtrack(node.right, level+1)
            rs[len(rs) - level - 1].append(node.val)
        rs = []
        backtrack(root)
        return rs

相似问题

  1. Binary Tree Level Order Traversal
  2. Average of Levels in Binary Tree
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