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12. Maximum Depth of Binary Tree

easyAsked at Canva

Find the maximum depth of a binary tree — Canva uses this to verify simple recursion sense around bounding deeply nested group depth.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the root of a binary tree, return its maximum depth — the number of nodes along the longest path from the root to a leaf.

Constraints

  • 0 <= nodes <= 10^4
  • -100 <= Node.val <= 100

Examples

Example 1

Input
root=[3,9,20,null,null,15,7]
Output
3

Example 2

Input
root=[1,null,2]
Output
2

Approaches

1. BFS level count

Level-order traversal incrementing depth per level.

Time
O(n)
Space
O(n)
if (!root) return 0;
let depth = 0, q = [root];
while (q.length) {
  const next = [];
  for (const n of q) { if (n.left) next.push(n.left); if (n.right) next.push(n.right); }
  q = next; depth++;
}
return depth;

Tradeoff:

2. DFS recursion

Depth of any subtree = 1 + max(left, right). Short, classic.

Time
O(n)
Space
O(h)
function maxDepth(root) {
  if (!root) return 0;
  return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
}

Tradeoff:

Canva-specific tips

Canva interviewers like both recursive and iterative versions side by side — it shows you understand when call-stack depth could matter for huge designs.

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