102. Binary Tree Level Order Traversal (Medium)

Description:

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

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

---

Solution:

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if root == None:
            return []
        
        node_ls = [root]
        value_ls = []
        value_cache = [root.val]
        
        def helper(node_ls):
            cache = []
            for node in node_ls:
                if node != None:
                    cache.append(node.left)
                    cache.append(node.right)
            return cache
            
        while value_cache:
            value_ls.append(value_cache)
            node_ls = helper(node_ls)
            value_cache = [n.val for n in node_ls if n != None]
            
        return value_ls

Runtime: 40 ms, faster than 82.05% of Python3 online submissions for Binary Tree Level Order Traversal.
Memory Usage: 13.5 MB, less than 39.42% of Python3 online submissions for Binary Tree Level Order Traversal.