feat: Add solution for LeetCode problem 128

Author: WhiteHyun

longest-consecutive-sequence/WhiteHyun.swift

@@ -0,0 +1,25 @@
+//  
+//  128. Longest Consecutive Sequence
+//  https://leetcode.com/problems/longest-consecutive-sequence/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/01.
+//  
+
+final class Solution {
+  func longestConsecutive(_ nums: [Int]) -> Int {
+    let set = Set(nums)
+    var best = 0
+    for number in set where !set.contains(number - 1) {
+      var next = number + 1
+      while set.contains(next) {
+        next += 1
+      }
+      if best < next - number {
+        best = next - number
+      }
+    }
+
+    return best
+  }
+}