102. 二叉树的层序遍历

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        result,result_sub,list_ = [],[],[]
        if root: list_.append(root)
        while list_:
            size = len(list_)
            # print(size)
            while size:
                root = list_.pop(0)
                result_sub.append(root.val)
                size -= 1
                if root.left:
                    list_.append(root.left)
                if root.right:
                    list_.append(root.right) 
                # print(list_)
            result.append(result_sub)
            result_sub = []
        return result