96. Maximum Depth of Binary Tree

You're building a feature that needs to calculate the maximum depth of a nested data structure or tree. This is useful for validating nested forms, calculating DOM tree depth, or analyzing hierarchical data.

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.

Requirements

1. Basic Functionality

• Calculate maximum depth of binary tree
• Depth is number of nodes from root to deepest leaf
• Handle empty tree (return 0)
• Handle single node (return 1)

Example Usage

// Tree:     3
//          / \
//         9   20
//            /  \
//           15   7
maxDepth(root); // 3

// Tree: 1
maxDepth(root); // 1

// Tree: null
maxDepth(null); // 0

Real-World Context

This problem models real frontend scenarios:

• DOM tree depth: Calculate maximum nesting level
• Nested form validation: Check maximum nesting depth
• Tree visualization: Determine tree height for rendering
• Data structure analysis: Analyze hierarchical data depth