feat: Add solution for LeetCode problem 15

Author: WhiteHyun

3sum/WhiteHyun.swift

@@ -0,0 +1,41 @@
+//  
+//  15. 3Sum
+//  https://leetcode.com/problems/3sum/description/
+//  Dale-Study
+//  
+//  Created by WhiteHyun on 2024/06/04.
+//  
+
+class Solution {
+  func threeSum(_ nums: [Int]) -> [[Int]] {
+    var result: [[Int]] = []
+    let sorted = nums.sorted()
+
+    for (index, element) in sorted.enumerated() where index <= 0 || element != sorted[index - 1] {
+      var left = index + 1
+      var right = sorted.count - 1
+
+      while left < right {
+        let threeSum = element + sorted[left] + sorted[right]
+        if threeSum > 0 {
+          right -= 1
+          continue
+        }
+        if threeSum < 0 {
+          left += 1
+          continue
+        }
+
+        result.append([element, sorted[left], sorted[right]])
+        left += 1
+
+        while sorted[left] == sorted[left - 1] && left < right {
+          left += 1
+        }
+      }
+    }
+
+
+    return result
+  }
+}