[Swift]LeetCode225. 用队列实现栈 | Implement Stack using Queues

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Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

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

---

使用队列实现栈的下列操作：

• push(x) -- 元素 x 入栈
• pop() -- 移除栈顶元素
• top() -- 获取栈顶元素
• empty() -- 返回栈是否为空

注意:

• 你只能使用队列的基本操作-- 也就是 push to back, peek/pop from front, size, 和 is empty 这些操作是合法的。
• 你所使用的语言也许不支持队列。 你可以使用 list 或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。
• 你可以假设所有操作都是有效的（例如, 对一个空的栈不会调用 pop 或者 top 操作）。

---

8ms

class MyStack {

    var stack: Array<Int>
    
    /** Initialize your data structure here. */
    init() {
        stack = []
    }
    
    /** Push element x onto stack. */
    func push(_ x: Int) {
        stack.append(x)
    }
    
    /** Removes the element on top of the stack and returns that element. */
    func pop() -> Int {
        return stack.removeLast()
    }
    
    /** Get the top element. */
    func top() -> Int {
        return stack.last!
    }
    
    /** Returns whether the stack is empty. */
    func empty() -> Bool {
        return stack.isEmpty
    }

}

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

---

8ms

class MyStack {

    fileprivate var array : [Int]
    
    /** Initialize your data structure here. */
    init() {
        array = [Int]()
    }
    
    /** Push element x onto stack. */
    func push(_ x: Int) {
        array.append(x)
    }
    
    /** Removes the element on top of the stack and returns that element. */
    func pop() -> Int {
        return array.removeLast()
    }
    
    /** Get the top element. */
    func top() -> Int {
        return array.last!
    }
    
    /** Returns whether the stack is empty. */
    func empty() -> Bool {
        return array.isEmpty
    }
}

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

---

12ms

class MyStack {

    private var queue = [Int]()

    /** Initialize your data structure here. */
    init() {

    }

    /** Push element x onto stack. */
    func push(_ x: Int) {
        queue.append(x)
        for _ in 0..<queue.count - 1 {
            queue.append(queue.removeFirst())
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    func pop() -> Int {
        return queue.removeFirst()
    }

    /** Get the top element. */
    func top() -> Int {
        return queue.first!
    }

    /** Returns whether the stack is empty. */
    func empty() -> Bool {
        return queue.isEmpty
    }
}

---

16ms

//: [Previous](@previous)

import Foundation


class MyQueue {
    
    var array = Array<Int>()
    
    /** Initialize your data structure here. */
    init() {
        array = []
    }
    
    /** Push element x to the back of queue. */
    func push(_ x: Int) {
        array.append(x)
    }
    
    /** Removes the element from in front of queue and returns that element. */
    func pop() -> Int {
        let last = array.first!
        array.remove(at: 0)
        return last
    }
    
    /** Get the front element. */
    func peek() -> Int {
        return array.first!
    }
    
    /** Returns whether the queue is empty. */
    func empty() -> Bool {
        return array.count == 0
    }
    
    func size() -> Int {
        
        return array.count;
    }
}

class MyStack {
    
    var queue: MyQueue?
    var helpQueue: MyQueue?
    /** Initialize your data structure here. */
    
    init() {
        queue = MyQueue.init()
        helpQueue = MyQueue.init()
    }
    
    /** Push element x onto stack. */
    func push(_ x: Int) {
        queue?.push(x)
    }
    
    /** Removes the element on top of the stack and returns that element. */
    func pop() -> Int {
        
        shift()
        let popObj = queue?.pop()
        swap()
        return popObj!
    }
    
    /** Get the top element. */
    func top() -> Int {
        shift()
        let popObj = queue!.peek()
        helpQueue?.push(queue!.pop())
        swap()
        return popObj
    }
    
    /** Returns whether the stack is empty. */
    func empty() -> Bool {
        
        return queue!.empty() && helpQueue!.empty();
    }
    
    func swap() {
        
        (queue, helpQueue) = (helpQueue, queue)
    }
    
    func shift() {
        
        while queue?.size() != 1 {
            
            helpQueue?.push(queue!.pop())
        }
    }
    
}

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