0102: Binary Tree Level Order Traversal

Problem Statement

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

Code Solution

from collections import deque

class Solution:
    def levelOrder(self, root):
	    
        if not root:
            return []
        
        dq = deque([root])
        
        out = []
        while dq:
            N = len(dq)
            temp = []
            for _ in range(N):
                curr = dq.popleft()
                temp.append(curr.val)
                if curr.left: dq.append(curr.left)
                if curr.right: dq.append(curr.right)
            out.append(temp)
        return out