03.04. Implement Queue using Stacks

Description

Implement a MyQueue class which implements a queue using two stacks.

 

Example:

MyQueue queue = new MyQueue();



queue.push(1);

queue.push(2);

queue.peek();  // return 1

queue.pop();   // return 1

queue.empty(); // return 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).

 

Solutions

Solution 1: Double Stack

We use two stacks, where stk1 is used for enqueue, and another stack stk2 is used for dequeue.

When enqueueing, we directly push the element into stk1. The time complexity is \(O(1)\).

When dequeueing, we first check whether stk2 is empty. If it is empty, we pop all elements from stk1 and push them into stk2, and then pop an element from stk2. If stk2 is not empty, we directly pop an element from stk2. The amortized time complexity is \(O(1)\).

When getting the front element, we first check whether stk2 is empty. If it is empty, we pop all elements from stk1 and push them into stk2, and then get the top element from stk2. If stk2 is not empty, we directly get the top element from stk2. The amortized time complexity is \(O(1)\).

When checking whether the queue is empty, we only need to check whether both stacks are empty. The time complexity is \(O(1)\).

Python3JavaC++GoTypeScriptRustSwift

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()

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();
 */

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();
 */

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();
 */

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()
 */

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);
            }
        }
    }
}

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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