[Search (BFS)] Jianzhi Offer 32 - I. Print binary tree from top to bottom

Title description :
Print each node of the binary tree from top to bottom, and the nodes of the same layer are printed from left to right.

Example :

Problem solution : This problem is a typical BFS, so you can use the template directly. Queue+while queue

# 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[int]:
        #BFS
        from collections import deque
        if not root:
            return []
        queue = deque()
        queue.append(root)
        result = []
        while queue:
            pre = queue.popleft()
            result.append(pre.val)
            if pre.left:
                queue.append(pre.left)
            if pre.right:
                queue.append(pre.right)
        return result