swift: add «344. Reverse String»

Signed-off-by: Matej Focko <me@mfocko.xyz>

swift/reverse-string.swift

@@ -0,0 +1,8 @@
+class Solution {
+    func reverseString(_ s: inout [Character]) {
+        for i in 0..<(s.count / 2) {
+            let j = s.count - i - 1
+            (s[i], s[j]) = (s[j], s[i])
+        }
+    }
+}