0232-Implement Queue using Stacks.md

Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Solution

/*
 ### Implement Queue using Stacks
 
 Implement the following operations of a queue using stacks.
 
 * __push(x)__ -- Push element x to the back of queue.
 * __pop()__ -- Removes the element from in front of queue.
 * __peek()__ -- Get the front element.
 * __empty()__ -- Return whether the queue is empty.
 
 __Example:__

MyQueue queue = new MyQueue();

queue.push(1); queue.push(2); queue.peek(); // returns 1 queue.pop(); // returns 1 queue.empty(); // returns false

__Notes:__
* You must use only standard operations of a stack -- which means only `push to top`,  `peek/pop from top`, `size`, and `is empty` operations are valid.
* Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
* You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
*/

struct Stack<Element> {
   private var storage = Array<Element>()
   
   mutating func push(_ element: Element) {
       storage.append(element)
   }
   
   mutating func pop() -> Element? {
       storage.popLast()
   }
   
   var isEmpty: Bool {
       storage.isEmpty
   }
   
   var top: Element? {
       storage.last
   }
   
   var size: Int {
       storage.count
   }
}



struct Queue<Element> {
   private var storage = Stack<Element>()
   // O(1)
   var peek: Element? {
       storage.top
   }
   // O(1)
   mutating func dequeue() -> Element? {
       storage.pop()
   }
   // O(1)
   var isEmpty: Bool {
       peek == nil
   }
   
   // Space O(n), Time: O(n + n)
   mutating func enqueue(_ element: Element) {
       var stack = Stack<Element>()
       while let pop = storage.pop() {
           stack.push(pop)
       }
       stack.push(element)
       
       while let pop = stack.pop() {
           storage.push(pop)
       }
   }
}

var queue = Queue<Int>()

queue.enqueue(1) // 1
queue.peek 		// 1
queue.enqueue(2) 
queue.peek 	   // 1
queue.dequeue() // 1
queue.dequeue() // 2

/**
* Your MyQueue object will be instantiated and called as such:
* let obj = MyQueue()
* obj.push(x)
* let ret_2: Int = obj.pop()
* let ret_3: Int = obj.peek()
* let ret_4: Bool = obj.empty()
*/