feat: add swift implementation to lcof problem: No.21 (doocs#2880)

Author: klever34

Diff for: lcof/面试题21. 调整数组顺序使奇数位于偶数前面/README.md

@@ -182,6 +182,28 @@ public class Solution {
 }
 ```
 
+#### Swift
+
+```swift
+class Solution {
+    func exchange(_ nums: [Int]) -> [Int] {
+        var nums = nums
+        var j = 0
+        
+        for i in 0..<nums.count {
+            if nums[i] % 2 == 1 {
+                let temp = nums[i]
+                nums[i] = nums[j]
+                nums[j] = temp
+                j += 1
+            }
+        }
+        
+        return nums
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

Diff for: lcof/面试题21. 调整数组顺序使奇数位于偶数前面/Solution.swift

@@ -0,0 +1,17 @@
+class Solution {
+    func exchange(_ nums: [Int]) -> [Int] {
+        var nums = nums
+        var j = 0
+        
+        for i in 0..<nums.count {
+            if nums[i] % 2 == 1 {
+                let temp = nums[i]
+                nums[i] = nums[j]
+                nums[j] = temp
+                j += 1
+            }
+        }
+        
+        return nums
+    }
+}