feat: Add solution for LeetCode problem 104

Author: WhiteHyun

maximum-depth-of-binary-tree/WhiteHyun.swift

@@ -0,0 +1,30 @@
+//  
+//  104. Maximum Depth of Binary Tree
+//  https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/05/12.
+//  
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+final class Solution {
+  func maxDepth(_ node: TreeNode?) -> Int {
+    guard let node else { return 0 }
+
+    return max(maxDepth(node.left), maxDepth(node.right)) + 1
+  }
+}