剑指 Offer 32

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如:
给定二叉树: [3,9,20,null,null,15,7],

3
/
9 20
/
15 7
返回其层次遍历结果：

[
[3],
[9,20],
[15,7]
]
 

提示：

节点总数 <= 1000

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        queue,res=[root],[]
        while queue:
            n=len(queue)
            res.append([])
            for i in range(n):
                node=queue[i]
                res[-1].append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            queue=queue[n:]
        return res

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        res=[]
        def add_to_res(level,node):
            if level>len(res)-1:
                res.append([])
            res[level].append(node.val)
            if node.left:
                add_to_res(level+1,node.left)
            if node.right:
                add_to_res(level+1,node.right)
        
        add_to_res(0,root)
        return res