Leetcode Problem No : 104 -> Maximum Depth of Binary Tree

# 🌳 Maximum Depth of Binary Tree (LeetCode #104)

## 📌 Problem Statement  
Given the `root` of a binary tree, return its **maximum depth**.  
The maximum depth is the number of nodes along the longest path from the root node down to the **furthest leaf node**.

### 🔹 Example 1:
```python
Input: root = [3,9,20,null,null,15,7]  
Output: 3
```
```
    3
   / \
  9  20
    /  \
   15   7
```

### 🔹 Example 2:
```python
Input: root = [1, null, 2]  
Output: 2
```

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## 🚀 Solution Approach  

### 1️⃣ **Depth-First Search (DFS - Recursive):**  
   - Recursively find the depth of left and right subtrees.  
   - Maximum depth = `1 + max(left_depth, right_depth)`.  

### 2️⃣ **Breadth-First Search (BFS - Iterative):**  
   - Use a **queue** to traverse level by level.  
   - Count the number of levels in the tree.

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## 📝 Python Solution (Recursive - DFS)
```python
# """class Solution:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right"""

class Solution(object):
    def maxDepth(self, root):
        if root is None:
            return 0
        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
---

## ⏳ Complexity Analysis  
✅ **Time Complexity:** `O(N)` (Visit all nodes once)  
✅ **Space Complexity:**  
   - **DFS Recursive:** `O(N)` (Recursion stack in worst case)  
   - **BFS Iterative:** `O(N)` (Queue for level-order traversal)

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