## 1. Problem

104. Maximum Depth of Binary Tree

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.

Example 1:

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

Example 2:

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

Constraints:

- The number of nodes in the tree is in the range [0, 10^4].
- -100 <= Node.val <= 100

## 2. Solution

I solved this problem like this.

- Complexity
- Time complexity : O(N)
- Space complexity : O(N)

- Step
- Using list, append node and depth.
- Confirm data and push left, right children while list is empty.

```
class Solution:
def maxDepth(self, root: Optional[TreeNode]) -> int:
answer = 0
data = [(root, 1)]
if not root:
return 0
while data:
node, depth = data.pop()
answer = max(answer, depth)
if node.left:
data.append((node.left, depth + 1))
if node.right:
data.append((node.right, depth + 1))
return answer
```