LeetCode107 Binary Tree Level Order Traversal II

最新推荐文章于 2019-05-09 23:27:00 发布

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本文介绍了一种从叶节点到根节点的层次遍历二叉树的方法，并提供了一个具体的Python实现示例。

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],

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

<思路>记录一层的节点，存储这层节点的val，用BFS继续遍历下一层节点。

# 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 is None:
            return []
        
        level = [root]              #store nodes for each level
        final = [[root.val]]        #append vals
       
        while level:     

            val=[]                   #store val for each level
            queue=[]

            for node in level:
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            for node in queue:
                val.append(node.val)

            if val:
                final.append(val)
            level = queue
            
        return final[::-1]