Binary Tree Level Order Traversal

Problem description
Level: medium
Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

Given binary tree {3,9,20,#,#,15,7},


return its level order traversal as:


Using only 1 queue to implement it.


There are multiple approaches to solve the problem.
1. two queues ( queue 1 save one level, then find nodes for next level, save in to queue 2, then clean queue 1, find next level nodes save them in queue 1)
2. one queue + dummy number (use # as level break, 3 # 9 20 # 15 7)
3. one queue
4. no queue

Solution 1 (no queue)

Solution 2 – one queue

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