Free AI LeetCode Communication Coach — Maximum Depth of Binary Tree

The problem

Given the root of a binary tree, return its maximum depth.

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

Base case first.

def maxDepth(root):
    if not root:
        return 0
    return 1 + max(maxDepth(root.left), maxDepth(root.right))

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 …”

• “If the node is null, depth is zero.”
• “Depth is one plus the max of left and right.”

Practise out loud

1. 1. Restate

   I need the number of nodes on the longest root-to-leaf path.

2. 2. Approach

   I will use DFS. If the node is null, depth is zero. Otherwise depth is one plus the max of the left and right depths.

3. 3. Example

   On a root with left leaf and a right subtree of height two, the answer is three.

4. 4. Time and space

   Time is O(n). Space is O(h) for the recursion stack.

5. 5. Edge cases

   I would check an empty tree and a skewed tree that is basically a linked list.

Common mistakes when explaining Maximum Depth of Binary Tree

• Forgetting the null base case out loud.
• Saying O(1) space when recursion uses O(h).