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面试题 03.04. 化栈为队

题目描述

实现一个MyQueue类，该类用两个栈来实现一个队列。


示例：

MyQueue queue = new MyQueue();

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


说明：

  • 你只能使用标准的栈操作 -- 也就是只有 push to top, peek/pop from top, size 和 is empty 操作是合法的。
  • 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，只要是标准的栈操作即可。
  • 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

解法

方法一：双栈

我们使用两个栈，其中栈 stk1用于入队，另一个栈 stk2 用于出队。

入队时，直接将元素入栈 stk1。时间复杂度 \(O(1)\)。

出队时，先判断栈 stk2 是否为空，如果为空，则将栈 stk1 中的元素全部出栈并入栈 stk2，然后再从栈 stk2 中出栈一个元素。如果栈 stk2 不为空，则直接从栈 stk2 中出栈一个元素。均摊时间复杂度 \(O(1)\)。

获取队首元素时，先判断栈 stk2 是否为空，如果为空，则将栈 stk1 中的元素全部出栈并入栈 stk2，然后再从栈 stk2 中获取栈顶元素。如果栈 stk2 不为空，则直接从栈 stk2 中获取栈顶元素。均摊时间复杂度 \(O(1)\)。

判断队列是否为空时，只要判断两个栈是否都为空即可。时间复杂度 \(O(1)\)。

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class MyQueue:
    def __init__(self):
        self.stk1 = []
        self.stk2 = []

    def push(self, x: int) -> None:
        self.stk1.append(x)

    def pop(self) -> int:
        self.move()
        return self.stk2.pop()

    def peek(self) -> int:
        self.move()
        return self.stk2[-1]

    def empty(self) -> bool:
        return not self.stk1 and not self.stk2

    def move(self):
        if not self.stk2:
            while self.stk1:
                self.stk2.append(self.stk1.pop())


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

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class MyQueue {
    private Deque<Integer> stk1 = new ArrayDeque<>();
    private Deque<Integer> stk2 = new ArrayDeque<>();

    public MyQueue() {
    }

    public void push(int x) {
        stk1.push(x);
    }

    public int pop() {
        move();
        return stk2.pop();
    }

    public int peek() {
        move();
        return stk2.peek();
    }

    public boolean empty() {
        return stk1.isEmpty() && stk2.isEmpty();
    }

    private void move() {
        while (stk2.isEmpty()) {
            while (!stk1.isEmpty()) {
                stk2.push(stk1.pop());
            }
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

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class MyQueue {
public:
    MyQueue() {
    }

    void push(int x) {
        stk1.push(x);
    }

    int pop() {
        move();
        int ans = stk2.top();
        stk2.pop();
        return ans;
    }

    int peek() {
        move();
        return stk2.top();
    }

    bool empty() {
        return stk1.empty() && stk2.empty();
    }

private:
    stack<int> stk1;
    stack<int> stk2;

    void move() {
        if (stk2.empty()) {
            while (!stk1.empty()) {
                stk2.push(stk1.top());
                stk1.pop();
            }
        }
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */

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type MyQueue struct {
    stk1 []int
    stk2 []int
}

func Constructor() MyQueue {
    return MyQueue{[]int{}, []int{}}
}

func (this *MyQueue) Push(x int) {
    this.stk1 = append(this.stk1, x)
}

func (this *MyQueue) Pop() int {
    this.move()
    ans := this.stk2[len(this.stk2)-1]
    this.stk2 = this.stk2[:len(this.stk2)-1]
    return ans
}

func (this *MyQueue) Peek() int {
    this.move()
    return this.stk2[len(this.stk2)-1]
}

func (this *MyQueue) Empty() bool {
    return len(this.stk1) == 0 && len(this.stk2) == 0
}

func (this *MyQueue) move() {
    if len(this.stk2) == 0 {
        for len(this.stk1) > 0 {
            this.stk2 = append(this.stk2, this.stk1[len(this.stk1)-1])
            this.stk1 = this.stk1[:len(this.stk1)-1]
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Peek();
 * param_4 := obj.Empty();
 */

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class MyQueue {
    stk1: number[];
    stk2: number[];

    constructor() {
        this.stk1 = [];
        this.stk2 = [];
    }

    push(x: number): void {
        this.stk1.push(x);
    }

    pop(): number {
        this.move();
        return this.stk2.pop();
    }

    peek(): number {
        this.move();
        return this.stk2.at(-1);
    }

    empty(): boolean {
        return !this.stk1.length && !this.stk2.length;
    }

    move(): void {
        if (!this.stk2.length) {
            while (this.stk1.length) {
                this.stk2.push(this.stk1.pop()!);
            }
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * var obj = new MyQueue()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.peek()
 * var param_4 = obj.empty()
 */

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use std::collections::VecDeque;

struct MyQueue {
    stk1: Vec<i32>,
    stk2: Vec<i32>,
}

impl MyQueue {
    fn new() -> Self {
        MyQueue {
            stk1: Vec::new(),
            stk2: Vec::new(),
        }
    }

    fn push(&mut self, x: i32) {
        self.stk1.push(x);
    }

    fn pop(&mut self) -> i32 {
        self.move_elements();
        self.stk2.pop().unwrap()
    }

    fn peek(&mut self) -> i32 {
        self.move_elements();
        *self.stk2.last().unwrap()
    }

    fn empty(&self) -> bool {
        self.stk1.is_empty() && self.stk2.is_empty()
    }

    fn move_elements(&mut self) {
        if self.stk2.is_empty() {
            while let Some(element) = self.stk1.pop() {
                self.stk2.push(element);
            }
        }
    }
}

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class MyQueue {
    private var stk1: [Int] = []
    private var stk2: [Int] = []

    init() {}

    func push(_ x: Int) {
        stk1.append(x)
    }

    @discardableResult
    func pop() -> Int {
        move()
        return stk2.removeLast()
    }

    func peek() -> Int {
        move()
        return stk2.last!
    }

    func empty() -> Bool {
        return stk1.isEmpty && stk2.isEmpty
    }

    private func move() {
        if stk2.isEmpty {
            while !stk1.isEmpty {
                stk2.append(stk1.removeLast())
            }
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * let obj = new MyQueue();
 * obj.push(x);
 * let param_2 = obj.pop();
 * let param_3 = obj.peek();
 * var myValue : Bool
 * myValue = obj.empty();
 */

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