Free AI LeetCode Communication Coach — Binary Tree Level Order Traversal

The problem

Return the level-order traversal of a binary tree — values level by level.

Example: tree [3,9,20,null,null,15,7] → [[3],[9,20],[15,7]]

Say “level size” explicitly.

from collections import deque
if not root:
    return []
q = deque([root])
res = []
while q:
    level = []
    for _ in range(len(q)):
        node = q.popleft()
        level.append(node.val)
        if node.left: q.append(node.left)
        if node.right: q.append(node.right)
    res.append(level)
return res

How to explain it

1. 1. Restate

   Say the problem in your own words.

   One or two sentences. Show you understood the input, the output, and the goal — not that you memorised the prompt.

2. 2. Approach

   Name the method before you code.

   Brute force first if you need it, then the structure you will use: hash map, two pointers, stack, binary search.

3. 3. Example

   Walk one concrete input.

   Pick small numbers. Say what you store, what you compare, and what you return. Interviewers follow an example more easily than abstract talk.

4. 4. Time and space

   One sentence each.

   After the example, before you claim you are done. “Time is O(n) because we scan once. Space is O(n) for the map.”

5. 5. Edge cases

   Name at least one unusual input.

   Empty input, duplicates, already sorted, overflow. Invite a follow-up: “I would also check …”

• “I BFS and process the queue length as one level.”
• “I push children only after I finish the current level count.”

Practise out loud

1. 1. Restate

   I need each level’s values grouped in a list from top to bottom.

2. 2. Approach

   I will BFS with a queue. For each level I note how many nodes are in the queue, then process exactly that many and push their children.

3. 3. Example

   Root 3 is level zero. Then 9 and 20. Then 15 and 7. I append each level’s list to the result.

4. 4. Time and space

   Time is O(n). Space is O(n) for the queue in the widest level.

5. 5. Edge cases

   I would check an empty tree and a single-node tree.

Common mistakes when explaining Binary Tree Level Order Traversal

• Not capturing level size before the inner loop.
• Confusing DFS preorder with level order.