102-binary-tree-level-order-traversal

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Description

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).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000

Solution(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]]:
        if not root:
            return []
        q = deque([root])
        res = []
        while q:
            cur_level = []
            n = len(q)
            for _ in range(n):
                node = q.popleft()
                if node:
                    cur_level.append(node.val)
                    if node.left:
                        q.append(node.left)
                    if node.right:
                        q.append(node.right)
            res.append(cur_level)
        return res