# 104. Maximum Depth of Binary Tree

## The Problem

Link to original problem on Leetcode

Given a binary 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.

**Note**: A leaf is a node with no children.

## Example

Given binary tree `[3,9,20,null,null,15,7]`

,

```
3
/ \
9 20
/ \
15 7
```

return its depth = 3.

## My Solution

This solution uses recursion.

`const maxDepth = (root) => {`

// Terminate immediately if there is no node provided

// This is handy when called on a non-existant child.

if (!root) {

return 0;

}

// Recursively call this function on the children of the node.

// Add a 1, because if this is a leaf, you'd get Math.max(0, 0).

// The 1 represents this level, which will serve as the starting

// point to count each level back to the root.

return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;

}

## Others' Solutions

Solutions page locked, but here is another JS answer that is roughly equivalent to mine. Could be a one-liner, but I find it easier to read as three.

`const maxDepth = (root) => !root`

? 0

: 1 + Math.max(maxDepth(root.left), maxDepth(root.right));

Another solution that uses iteration instead of recursion:

`const maxDepth = (root) => {`

if (!root) return 0;

// Initialize a queue with the root node.

const queue = [root];

// Initialize the tree's depth at 0.

let depth = 0;

while (queue.length !== 0) {

const len = queue.length;

// Iterates over each item in the queue, adding all child nodes

// to the queue. So in the original example, the queue starts at

// [3] and becomes [9, 20]. Then the depth is iterated by 1.

// Next would be [9, 20] to [15, 7].

// Loop ended when there's nothing left to add to the queue.

for (let i = 0; i < len; i++) {

let item = queue.shift()

if (item.left) queue.push(item.left);

if (item.right) queue.push(item.right);

}

depth++;

}

// Return the deepest level you could hit.

return depth;

};