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Java-based LeetCode algorithm problem solutions, regularly updated

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104\. Maximum Depth of Binary Tree

Easy

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:**

![](https://assets.leetcode.com/uploads/2020/11/26/tmp-tree.jpg)

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

**Output:** 3 

**Example 2:**

**Input:** root = [1,null,2]

**Output:** 2 

**Example 3:**

**Input:** root = []

**Output:** 0 

**Example 4:**

**Input:** root = [0]

**Output:** 1 

**Constraints:**

*   The number of nodes in the tree is in the range [0, 10⁴].
*   `-100 <= Node.val <= 100`

To solve the "Maximum Depth of Binary Tree" problem in Java with a `Solution` class, we'll perform a depth-first search (DFS) traversal of the binary tree. Below are the steps:

1. **Create a `Solution` class**: Define a class named `Solution` to encapsulate our solution methods.

2. **Create a `maxDepth` method**: This method takes the root node of the binary tree as input and returns its maximum depth.

3. **Check for null root**: Check if the root is null. If it is, return 0 as the depth.

4. **Perform DFS traversal**: Recursively compute the depth of the left and right subtrees. The maximum depth of the binary tree is the maximum depth of its left and right subtrees, plus 1 for the current node.

5. **Return the result**: After the DFS traversal is complete, return the maximum depth of the binary tree.

Here's the Java implementation:

```java
class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) return 0; // Check for empty tree
        int leftDepth = maxDepth(root.left); // Compute depth of left subtree
        int rightDepth = maxDepth(root.right); // Compute depth of right subtree
        return Math.max(leftDepth, rightDepth) + 1; // Return maximum depth of left and right subtrees, plus 1 for the current node
    }
    
    // Definition for a TreeNode
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        
        TreeNode() {}
        TreeNode(int val) { this.val = val; }
        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
}
```

This implementation follows the steps outlined above and efficiently computes the maximum depth of the binary tree in Java using DFS traversal.