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➤微信公众号：山青咏芝（shanqingyongzhi）
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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, andis emptyoperations 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
1 class MyStack {
2
3 var stack: Array<Int>
4
5 /** Initialize your data structure here. */
6 init() {
7 stack = []
8 }
9
10 /** Push element x onto stack. */
11 func push(_ x: Int) {
12 stack.append(x)
13 }
14
15 /** Removes the element on top of the stack and returns that element. */
16 func pop() -> Int {
17 return stack.removeLast()
18 }
19
20 /** Get the top element. */
21 func top() -> Int {
22 return stack.last!
23 }
24
25 /** Returns whether the stack is empty. */
26 func empty() -> Bool {
27 return stack.isEmpty
28 }
29
30 }
31
32 /**
33 * Your MyStack object will be instantiated and called as such:
34 * let obj = MyStack()
35 * obj.push(x)
36 * let ret_2: Int = obj.pop()
37 * let ret_3: Int = obj.top()
38 * let ret_4: Bool = obj.empty()
39 */
8ms
1 class MyStack {
2
3 fileprivate var array : [Int]
4
5 /** Initialize your data structure here. */
6 init() {
7 array = [Int]()
8 }
9
10 /** Push element x onto stack. */
11 func push(_ x: Int) {
12 array.append(x)
13 }
14
15 /** Removes the element on top of the stack and returns that element. */
16 func pop() -> Int {
17 return array.removeLast()
18 }
19
20 /** Get the top element. */
21 func top() -> Int {
22 return array.last!
23 }
24
25 /** Returns whether the stack is empty. */
26 func empty() -> Bool {
27 return array.isEmpty
28 }
29 }
30
31 /**
32 * Your MyStack object will be instantiated and called as such:
33 * let obj = MyStack()
34 * obj.push(x)
35 * let ret_2: Int = obj.pop()
36 * let ret_3: Int = obj.top()
37 * let ret_4: Bool = obj.empty()
38 */
39
12ms
1 class MyStack {
2
3 private var queue = [Int]()
4
5 /** Initialize your data structure here. */
6 init() {
7
8 }
9
10 /** Push element x onto stack. */
11 func push(_ x: Int) {
12 queue.append(x)
13 for _ in 0..<queue.count - 1 {
14 queue.append(queue.removeFirst())
15 }
16 }
17
18 /** Removes the element on top of the stack and returns that element. */
19 func pop() -> Int {
20 return queue.removeFirst()
21 }
22
23 /** Get the top element. */
24 func top() -> Int {
25 return queue.first!
26 }
27
28 /** Returns whether the stack is empty. */
29 func empty() -> Bool {
30 return queue.isEmpty
31 }
32 }
16ms
1 //: [Previous](@previous)
2
3 import Foundation
4
5
6 class MyQueue {
7
8 var array = Array<Int>()
9
10 /** Initialize your data structure here. */
11 init() {
12 array = []
13 }
14
15 /** Push element x to the back of queue. */
16 func push(_ x: Int) {
17 array.append(x)
18 }
19
20 /** Removes the element from in front of queue and returns that element. */
21 func pop() -> Int {
22 let last = array.first!
23 array.remove(at: 0)
24 return last
25 }
26
27 /** Get the front element. */
28 func peek() -> Int {
29 return array.first!
30 }
31
32 /** Returns whether the queue is empty. */
33 func empty() -> Bool {
34 return array.count == 0
35 }
36
37 func size() -> Int {
38
39 return array.count;
40 }
41 }
42
43 class MyStack {
44
45 var queue: MyQueue?
46 var helpQueue: MyQueue?
47 /** Initialize your data structure here. */
48
49 init() {
50 queue = MyQueue.init()
51 helpQueue = MyQueue.init()
52 }
53
54 /** Push element x onto stack. */
55 func push(_ x: Int) {
56 queue?.push(x)
57 }
58
59 /** Removes the element on top of the stack and returns that element. */
60 func pop() -> Int {
61
62 shift()
63 let popObj = queue?.pop()
64 swap()
65 return popObj!
66 }
67
68 /** Get the top element. */
69 func top() -> Int {
70 shift()
71 let popObj = queue!.peek()
72 helpQueue?.push(queue!.pop())
73 swap()
74 return popObj
75 }
76
77 /** Returns whether the stack is empty. */
78 func empty() -> Bool {
79
80 return queue!.empty() && helpQueue!.empty();
81 }
82
83 func swap() {
84
85 (queue, helpQueue) = (helpQueue, queue)
86 }
87
88 func shift() {
89
90 while queue?.size() != 1 {
91
92 helpQueue?.push(queue!.pop())
93 }
94 }
95
96 }
97
98 /**
99 * Your MyStack object will be instantiated and called as such:
100 * let obj = MyStack()
101 * obj.push(x)
102 * let ret_2: Int = obj.pop()
103 * let ret_3: Int = obj.top()
104 * let ret_4: Bool = obj.empty()
105 */
来源：https://www.cnblogs.com/strengthen/p/10204324.html