fix: add use case

Author: passerbyloo

Skip-List/SkipList.playground/Contents.swift

@@ -5,3 +5,16 @@ print("Hello, Swift 4!")
 
 // SkipList is ready for Swift 4.
 // TODO: Add Test
+
+let k = SkipList<Int, String>()
+k.insert(key: 10, data: "10")
+k.insert(key: 12, data: "12")
+k.insert(key: 13, data: "13")
+k.insert(key: 20, data: "20")
+k.insert(key: 24, data: "24")
+
+if let value = k.get(key: 20) {
+  print(value)
+} else {
+  print("not found!")
+}

Skip-List/SkipList.playground/Sources/SkipList.swift

@@ -212,7 +212,7 @@ extension SkipList {
     }
   }
 
-  func insert(key: Key, data: Payload) {
+  public func insert(key: Key, data: Payload) {
     if head != nil {
       if let node = findNode(key: key) {
         // replace, in case of key already exists.

Skip-List/SkipList.swift

@@ -212,7 +212,7 @@ extension SkipList {
     }
   }
 
-  func insert(key: Key, data: Payload) {
+  public func insert(key: Key, data: Payload) {
     if head != nil {
       if let node = findNode(key: key) {
         // replace, in case of key already exists.