Siqing Xu's solution

Chanllenge_Questions.playground/Contents.swift

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+//: Playground - noun: a place where people can play
+
+import Cocoa
+
+var str = "Hello, playground"
+func swapVariables(inout s1:String,inout _ s2:String){
+    let s3:String=s1
+    s1=s2
+    s2=s3
+}
+var changedString1 = "My name is David"
+var changedString2 = "My last name is Xu"
+swapVariables(&changedString1, &changedString2)
+print(changedString1)
+print(changedString2)
+
+func addMultipleString(words:String...)->String{
+    var newString:String = ""
+    for word in words{
+        newString+=word
+    }
+    if words.count==0{return "none"}
+    else {return newString}
+}
+print (addMultipleString("my ","name ","is ","David."))
+print (addMultipleString())
+
+
+func swap<T>(inout a:T, inout with b:T) { (a, b) = (b, a) }   //demo code 
+var a = "Marin", b = "Todorov"
+swap(&a, &b)
+[a, b]
+
+func countFrom(from:Int,_ to:Int){
+    print(from,terminator:"")
+    if from<to{
+        countFrom(from+1,to)
+    }
+}
+countFrom(1,5)
+
+func reverseText(a:String) -> String {
+    var b:String = a
+    var c:String
+    c.append(b.endIndex.predecessor())
+
+}
+

Chanllenge_Questions.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='osx'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>