Problem description

Problem on leetcode

Given a n-ary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).


Input: root = [1,null,3,2,4,null,5,6] Output: 3

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14] Output: 5


I've done a handful of tree-traversal problems at this point, and this one is screaming out to me as a breadth-first search problem. I believe I just need to find the final value of how many levels I travel in the BFS and return it.

I wrote a BFS algorithm in the zigzag level order traversal problem. The problem even references level order traversal in its problem statement, so I feel like I"m on the right track.

The two lessons I learned from zigging and zagging were:

  1. Implement a BFS with a queue
  2. Use a depth variable to indicate how far down I've gone.

Remembering this, I think I can express the solution in JavaScript like so:

var maxDepth = function(root) {
    if(!root) return 0;
    const queue = [[root, 0]]
    let result;
    while(queue.length) {
        let [node, depth] = queue.shift();
        if (node.children) {
            for (let i=0; i<node.children.length; i++) {
                queue.push([node.children[i], depth+1]);
        result = depth + 1;
    return result;

And it works! Again, a strong reminder that I need to keep practicing my BFS and DFS implementations. When I first saw this problem, I got flipped the wrong direction and thought I needed to do depth-first search because I was going down. But I didn't realize I was just counting the levels of hierarchy in the tree, which wants BFS. I also had to go back and really read through my zigzag solution to reverse engineer the correct algorithm.