feat: add a Two Sum solution in Swift language (doocs#473)

* add a Two Sum solution in Swift language
* update README_EN to include the swift solution to 0001.Two Sum

Co-authored-by: Avismara Hugoppalu <[email protected]>

Author: distressedrook

solution/0000-0099/0001.Two Sum/README.md

@@ -108,6 +108,29 @@ var twoSum = function (nums, target) {
 };
 ```
 
+### **Swift**
+
+```swift
+class Solution {
+    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
+        var map = [Int: Int]()
+        var i = 0
+        for num in nums {
+            map[num] = i
+            i = i + 1
+        }
+        i = 0
+        for num in nums {
+            if let otherIndex = map[target - num], otherIndex != i {
+                return [i, otherIndex]
+            }
+            i = i + 1
+        }
+        return []
+    }
+}
+```
+
 ### **...**
 
 ```

solution/0000-0099/0001.Two Sum/README_EN.md

@@ -93,7 +93,28 @@ var twoSum = function (nums, target) {
   return [];
 };
 ```
+### **Swift**
 
+```swift
+class Solution {
+    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
+        var map = [Int: Int]()
+        var i = 0
+        for num in nums {
+            map[num] = i
+            i = i + 1
+        }
+        i = 0
+        for num in nums {
+            if let otherIndex = map[target - num], otherIndex != i {
+                return [i, otherIndex]
+            }
+            i = i + 1
+        }
+        return []
+    }
+}
+```
 ### **...**
 
 ```

solution/0000-0099/0001.Two Sum/Solution.swift

@@ -0,0 +1,18 @@
+class Solution {
+    func twoSum(_ nums: [Int], _ target: Int) -> [Int] {
+        var map = [Int: Int]()
+        var i = 0
+        for num in nums {
+            map[num] = i
+            i = i + 1
+        }
+        i = 0
+        for num in nums {
+            if let otherIndex = map[target - num], otherIndex != i {
+                return [i, otherIndex]
+            }
+            i = i + 1
+        }
+        return []
+    }
+}