feat: add swift implementation to lcof2 problem: No.004 (#2974)

* Swift implementation for LCOF2 004

* Update README.md

* Update Solution.swift

---------

Co-authored-by: Libin YANG <contact@yanglibin.info>

Author: klever34

lcof2/剑指 Offer II 004. 只出现一次的数字/README.md

@@ -150,6 +150,25 @@ function singleNumber(nums: number[]): number {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func singleNumber(_ nums: [Int]) -> Int {
+        var ans: Int32 = 0
+        for i in 0..<32 {
+            var cnt = 0
+            for num in nums {
+                cnt += (num >> i) & 1
+            }
+            cnt %= 3
+            ans |= Int32(cnt) << i
+        }
+        return Int(ans)
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 004. 只出现一次的数字/Solution.swift

@@ -0,0 +1,14 @@
+class Solution {
+    func singleNumber(_ nums: [Int]) -> Int {
+        var ans: Int32 = 0
+        for i in 0..<32 {
+            var cnt = 0
+            for num in nums {
+                cnt += (num >> i) & 1
+            }
+            cnt %= 3
+            ans |= Int32(cnt) << i
+        }
+        return Int(ans)
+    }
+}