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11. Maximum Depth of Binary Tree

easyAsked at GitLab

Return the maximum depth of a binary tree from root to deepest leaf.

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 the farthest leaf.

Constraints

  • 0 <= nodes <= 10^4
  • -100 <= 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

Walk the tree level by level, increment depth per level.

Time
O(n)
Space
O(n)
let q=[root], d=0;
while (q.length && q[0]){
  d++;
  q = q.flatMap(n => [n.left,n.right].filter(Boolean));
}
return d;

Tradeoff:

2. Recursive DFS

Depth = 1 + max(depth(left), depth(right)). The cleanest one-liner and the answer GitLab expects.

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

Tradeoff:

GitLab-specific tips

GitLab interviewers nudge you toward the recursive form because it mirrors how their CI pipeline-stage graphs are measured for max critical-path depth across job dependencies.

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