3.Binary_Tree_Level_Order_Traversal.py

# Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

# 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: TreeNode) -> List[List[int]]:
        if not root:
            return []
        
        q = [root]
        next_q = []
        level = []
        result = []
        
        while q != []:
            for node in q:
                level.append(node.val)
                if node.left:
                    next_q.append(node.left)
                if node.right:
                    next_q.append(node.right)
                    
            result.append(level)
            level = []
            q = next_q
            next_q = []
            
        return result