Added Stack playground

Author: JovannyEspinal

Stack/Stack.playground/Contents.swift

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+/*
+
+Stack
+
+A stack is like an array but with limited functionality. You can only push to add a new element to the top of the stack, pop to remove the element from the top, and peek at the top element without popping it off.
+
+A stack gives you a LIFO or last-in first-out order. The element you pushed last is the first one to come off with the next pop. (A very similar data structure, the queue, is FIFO or first-in first-out.)
+
+Insertion - O(1)
+Deletion - O(1)
+
+
+*/
+
+import UIKit
+
+public struct Stack<T> {
+    private var array = [T]()
+    
+    public var count: Int {
+        return array.count
+    }
+    
+    public var isEmpty: Bool {
+        return array.isEmpty
+    }
+    
+    public mutating func push(element: T) {
+        array.append(element)
+    }
+    
+    public mutating func pop() -> T? {
+        if count == 0 {
+            return nil
+        } else {
+            return array.removeLast()
+        }
+    }
+    
+    public func peek() -> T? {
+        return array.last
+    }
+    
+}
+
+
+var stackOfNames = Stack(array: ["Carl", "Lisa", "Stephanie", "Jeff", "Wade"]) // Initialized queue
+
+stackOfNames.push("Mike") // Adds element to the end of the stack. Stack is now ["Carl", "Lisa", "Stephanie", "Jeff", "Wade", "Mike"]
+
+stackOfNames.pop() // Removes and returns the last element in the stack. Returns "Mike" and then removes it from the stack
+
+stackOfNames.peek() // Returns the first element in the stack. Returns "Wade" since "Mike" was popped on the previous line
+
+stackOfNames.isEmpty // Checks to see if the stack is empty. Returns "False" since the stack still has elements in it
+
+

Stack/Stack.playground/contents.xcplayground

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

Stack/Stack.playground/timeline.xctimeline

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+<?xml version="1.0" encoding="UTF-8"?>
+<Timeline
+   version = "3.0">
+   <TimelineItems>
+   </TimelineItems>
+</Timeline>