feat: Add solution for LeetCode problem 5

Author: WhiteHyun

longest-palindromic-substring/WhiteHyun.swift

@@ -0,0 +1,39 @@
+//  
+//  5. Longest Palindromic Substring
+//  https://leetcode.com/problems/longest-palindromic-substring/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/07/06.
+//  
+
+class Solution {
+  func longestPalindrome(_ s: String) -> String {
+    let array = Array(s)
+    if array.count == 1 { return s }
+
+    var start = 0
+    var maxLength = 1
+
+    func expandAroundCenter(_ left: Int, _ right: Int) -> Int {
+      var l = left
+      var r = right
+      while l >= 0, r < array.count, array[l] == array[r] {
+        l -= 1
+        r += 1
+      }
+      return r - l - 1
+    }
+
+    for index in array.indices {
+      let length1 = expandAroundCenter(index, index) // 홀수 길이
+      let length2 = expandAroundCenter(index, index + 1) // 짝수 길이
+      let length = max(length1, length2)
+
+      if length > maxLength {
+        start = index - (length - 1) / 2
+        maxLength = length
+      }
+    }
+    return String(array[start ..< start + maxLength])
+  }
+}