# Problem Definition

## Description

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]`,

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

return its depth = 3.

## Notes

1. [LeetCode discussion - Simple solution using Java](https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/555/discuss/34216/Simple-solution-using-Java)
1. [LeetCode discussion - Two Java Iterative solution DFS and BFS](https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/555/discuss/34195/Two-Java-Iterative-solution-DFS-and-BFS)