[Swift]LeetCode232. 用栈实现队列 | Implement Queue using Stacks

概述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(

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 1queue.pop();   // returns 1queue.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).

使用栈实现队列的下列 *** 作：

push(x) -- 将一个元素放入队列的尾部。 pop() -- 从队列首部移除元素。 peek() -- 返回队列首部的元素。 empty() -- 返回队列是否为空。

示例:

MyQueue queue = new MyQueue();queue.push(1);queue.push(2);  queue.peek();  // 返回 1queue.pop();   // 返回 1queue.empty(); // 返回 false

说明:

你只能使用标准的栈 *** 作 -- 也就是只有 push to top,和 is empty  *** 作是合法的。 你所使用的语言也许不支持栈。你可以使用 List 或者 deque（双端队列）来模拟一个栈，只要是标准的栈 *** 作即可。 假设所有 *** 作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek *** 作）。

8ms

 1 class MyQueue { 2      3     var input = [Int]() 4     var output = [Int]() 5      6     func push(_ x: Int) { 7         input.append(x) 8     } 9     10     func pop() -> Int {11         if output.isEmpty {12             inputToOutput()13         }14         return output.removeLast()15     }16     17     func inputToOutput() {18         while !input.isEmpty {19             output.append(input.removeLast())20         }21     }22     23     func peek() -> Int {24         if output.isEmpty {25             inputToOutput()26         }27         return output.last!28     }29     30     func empty() -> Bool {31         return input.isEmpty && output.isEmpty32     }33 }34 35 /**36  * Your MyQueue object will be instantiated and called as such:37  * let obj = MyQueue()38  * obj.push(x)39  * let ret_2: Int = obj.pop()40  * let ret_3: Int = obj.peek()41  * let ret_4: Bool = obj.empty()42  */43  

12ms

 1 class MyQueue { 2  3     private var oldStack: [Int] 4     private var newStack: [Int] 5     /** Initialize your data structure here. */ 6     init() { 7         oldStack = [Int]() 8         newStack = [Int]() 9     }10     11     /** Push element x to the back of queue. */12     func push(_ x: Int) {13       newStack.append(x)14     }15     16     /** Removes the element from in front of queue and returns that element. */17     func pop() -> Int {18                 shiftStack()19 20       return oldStack.removeLast()21     }22     23     /** Get the front element. */24     func peek() -> Int {25         shiftStack()26       return oldStack.last!27     }28     29     /** Returns whether the queue is empty. */30     func empty() -> Bool {31       return oldStack.isEmpty && newStack.isEmpty32     }33     private func shiftStack() {34         if oldStack.isEmpty {35             while !newStack.isEmpty {36              oldStack.append(newStack.removeLast())37             }38         }39     }40 }41 42 /**43  * Your MyQueue object will be instantiated and called as such:44  * let obj = MyQueue()45  * obj.push(x)46  * let ret_2: Int = obj.pop()47  * let ret_3: Int = obj.peek()48  * let ret_4: Bool = obj.empty()49  */50  

12ms

 1 class Stack { 2      3     private var nums: [Int] = [] 4      5     var isEmpty: Bool { 6         return nums.isEmpty 7     } 8      9     func push(_ x: Int) {10         nums.append(x)11     }12     13     func pop() -> Int? {14         return nums.popLast()15     }16     17     func peek() -> Int? {18         return nums.last19     }20     21 }22 23 class MyQueue {24     25     private let stack = Stack()26     private let stack2 = Stack()27 28     /** Initialize your data structure here. */29     init() {30         31     }32     33     /** Push element x to the back of queue. */34     func push(_ x: Int) {35         stack.push(x)36     }37     38     /** Removes the element from in front of queue and returns that element. */39     func pop() -> Int {40         if stack2.isEmpty {41             while let num = stack.pop() {42                 stack2.push(num)43             }44         }45         46         return stack2.pop() ?? 047     }48     49     /** Get the front element. */50     func peek() -> Int {51         if stack2.isEmpty {52             while let num = stack.pop() {53                 stack2.push(num)54             }55         }56         57         return stack2.peek() ?? 058     }59     60     /** Returns whether the queue is empty. */61     func empty() -> Bool {62       return stack.isEmpty && stack2.isEmpty63     }64 }65 66 /**67  * Your MyQueue object will be instantiated and called as such:68  * let obj = MyQueue()69  * obj.push(x)70  * let ret_2: Int = obj.pop()71  * let ret_3: Int = obj.peek()72  * let ret_4: Bool = obj.empty()73  */74  

16ms

 1 class MyQueue { 2  3     fileprivate var stack = [Int?]() 4     fileprivate var reserveStack = [Int?]() 5      6      7     /** Initialize your data structure here. */ 8     init() { 9         stack = [Int]()10         reserveStack = [Int]()11     }12     13     /** Push element x to the back of queue. */14     func push(_ x: Int) {15         if reserveStack.isEmpty{16             stack.append(x)17         }else{18             // 翻转reserveStack19             while !reserveStack.isEmpty {20                 stack.append(reserveStack.popLast()!)21             }22             stack.append(x)23         }24         25     }26     27     /** Removes the element from in front of queue and returns that element. */28     func pop() -> Int {29         30         // 原栈为空，说明数据翻转到reserve栈中31         if stack.isEmpty{32             return reserveStack.removeLast()!33         }34         35         // 翻转stack36         while !stack.isEmpty {37             reserveStack.append(stack.removeLast())38         }39         40         return reserveStack.removeLast()!41     }42     43     /** Get the front element. */44     func peek() -> Int {45        46         // 原栈为空，说明数据翻转到reserve栈中47         if stack.isEmpty{48             return reserveStack.last!!49         }50         51         // 翻转stack52         while !stack.isEmpty {53             reserveStack.append(stack.removeLast()!)54         }55         56         return reserveStack.last!!57     }58     59     /** Returns whether the queue is empty. */60     func empty() -> Bool {61         62         if stack.isEmpty && reserveStack.isEmpty {63             return true64         }65         66         return false67     }68 }69 70 /**71  * Your MyQueue object will be instantiated and called as such:72  * let obj = MyQueue()73  * obj.push(x)74  * let ret_2: Int = obj.pop()75  * let ret_3: Int = obj.peek()76  * let ret_4: Bool = obj.empty()77  */78  

20ms

 1 class MyQueue { 2  3    var queue : [Int] 4     /** Initialize your data structure here. */ 5     init() { 6         self.queue = [Int]() 7     } 8      9     /** Push element x to the back of queue. */10     func push(_ x: Int) {11         self.queue.insert(x,at: 0)12     }13     14     /** Removes the element from in front of queue and returns that element. */15     func pop() -> Int {16         return self.queue.removeLast()17     }18     19     /** Get the front element. */20     func peek() -> Int {21         return self.queue.last!22     }23     24     /** Returns whether the queue is empty. */25     func empty() -> Bool {26         return self.queue.isEmpty27     }28 }29 30 /**31  * Your MyQueue object will be instantiated and called as such:32  * let obj = MyQueue()33  * obj.push(x)34  * let ret_2: Int = obj.pop()35  * let ret_3: Int = obj.peek()36  * let ret_4: Bool = obj.empty()37  */38  

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