102. Binary Tree Level Order Traversal

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

Approach

Solution

# 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
from collections import deque


class Solution:
    # T: O(n) S: O(n)
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        res = []

        q = deque()
        q.append(root)

        while q:
            qLen = len(q)
            level = []
            for i in range(qLen):
                node = q.popleft()
                if node:
                    level.append(node.val)
                    q.append(node.left)
                    q.append(node.right)
            if level:
                res.append(level)

        return res

Explanation