feat: Add solution for LeetCode problem 424

Author: WhiteHyun

longest-repeating-character-replacement/WhiteHyun.swift

@@ -0,0 +1,30 @@
+//  
+//  424. Longest Repeating Character Replacement
+//  https://leetcode.com/problems/longest-repeating-character-replacement/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/08.
+//  
+
+final class Solution {
+  func characterReplacement(_ s: String, _ k: Int) -> Int {
+    var maxFrequency = 0
+    var i = 0
+    var counter: [Character: Int] = [:]
+    let array = Array(s)
+
+    for index in array.indices {
+      counter[array[index], default: 0] += 1
+      if maxFrequency < counter[array[index]]! {
+        maxFrequency = counter[array[index]]!
+      }
+
+      if index - i + 1 > maxFrequency + k {
+        counter[array[i]]! -= 1
+        i += 1
+      }
+    }
+
+    return array.count - i
+  }
+}