# 102. Binary Tree Level Order Traversal
#### Difficulty: Medium
link: https://leetcode.com/problems/binary-tree-level-order-traversal/

### 1. Iterative method
#### $O(n)$ runtime, $O(n)$ space

##### python

# 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 = []
        level = [root] if root is not None else []
        while level:
            result.append([node.val for node in level])
            next_level = []
            for node in level:
                for child in [node.left, node.right]:
                    if child is not None:
                        next_level.append(child)
            level = next_level
        
        return result

精簡版
##### python

# 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]]:
        result = []
        level = [root] if root is not None else []
        while level:
            result.append([n.val for n in level])
            level = [c for n in level for c in (n.left, n.right) if c is not None]
        
        return result

###### tags: `leetcode`