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232. Implement Queue using Stacks

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Description

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

 

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

Solutions

Python3

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.s1 = []
        self.s2 = []


    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.s1.append(x)


    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        self._move()
        return self.s2.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        self._move()
        return self.s2[-1]


    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return len(self.s1) + len(self.s2) == 0


    def _move(self):
        """
        Move elements from s1 to s2.
        """
        if len(self.s2) == 0:
            while len(self.s1) > 0:
                self.s2.append(self.s1.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()

Java

class MyQueue {

    private Deque<Integer> s1 = new ArrayDeque<>();
    private Deque<Integer> s2 = new ArrayDeque<>();

    /** Initialize your data structure here. */
    public MyQueue() {

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        move();
        return s2.pop();
    }

    /** Get the front element. */
    public int peek() {
        move();
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }

    /** Move elements from s1 to s2. */
    private void move() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) {
                s2.push(s1.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();
 */

TypeScript

class MyQueue {
    stack1: number[];
    stack2: number[];
    constructor() {
        this.stack1 = [];
        this.stack2 = [];
    }

    push(x: number): void {
        this.stack1.push(x);
    }

    pop(): number {
        if (!this.stack2.length) {
            while (this.stack1.length) {
                this.stack2.push(this.stack1.pop());
            }
        }
        return this.stack2.pop();
    }

    peek(): number {
        if (!this.stack2.length) {
            while (this.stack1.length) {
                this.stack2.push(this.stack1.pop());
            }
        }
        return this.stack2[this.stack2.length - 1];
    }

    empty(): boolean {
        return !this.stack1.length && !this.stack2.length;
    }
}

/**
 * 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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