Skip to main content

6. Maximum Depth of Binary Tree

easyAsked at Baidu

Return the length of the longest root-to-leaf path in a binary tree.

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

Problem

Given the root of a binary tree, return its maximum depth. A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Constraints

  • Number of nodes is in [0, 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

Run BFS and count how many levels you drain from the queue.

Time
O(n)
Space
O(w)
let q=root?[root]:[],d=0;
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;d++;}
return d;

Tradeoff:

2. Recursive DFS

Depth at each node is 1 plus the max depth of its children; pure recursion.

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

Tradeoff:

Baidu-specific tips

Baidu uses tree-depth measurements when limiting crawler-frontier expansion per host, so they expect a clean recursion plus an explicit note on stack-depth risk at production scale.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Maximum Depth of Binary Tree and other Baidu interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →