059 力扣刷题 102. 二叉树的层次遍历

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。

例如:
给定二叉树: [3,9,20,null,null,15,7],

image.png

# 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 levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root is None:
            return []
        res = []
        cur_nodes =[root] #当前层的节点
        next_nodes =[] #下一层的节点
        res.append([i.val for i in cur_nodes]) #将当前层的值从到到右的遍历并存储
        while cur_nodes or next_nodes:
            for node in cur_nodes:
                if node.left:
                    next_nodes.append(node.left)
                if node.right:
                    next_nodes.append(node.right)
            if next_nodes:
                res.append([i.val for i in next_nodes]) 
            cur_nodes = next_nodes
            next_nodes=[]
        return res