232. Implement Queue using Stacks (Easy)

https://leetcode.com/problems/implement-queue-using-stacks/

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

Solutions

class MyQueue {

    int size = 0;

    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();

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

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        size++;
        stack1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        for (int i = 0; i < size; i++) {
            stack2.push(stack1.pop());
        }

        size--;
        int ans = stack2.pop();

        for (int i = 0; i < size; i++) {
            stack1.push(stack2.pop());
        }

        return ans;
    }

    /** Get the front element. */
    public int peek() {
        for (int i = 0; i < size; i++) {
            stack2.push(stack1.pop());
        }

        int ans = stack2.peek();

        for (int i = 0; i < size; i++) {
            stack1.push(stack2.pop());
        }

        return ans;
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return size == 0;
    }
}

Incorrect Solutions

class MyQueue {

    // Wrong implementation of peek funciton

    int size = 0;

    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();

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

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        size++;
        stack1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        for (int i = 0; i < size; i++) {
            stack2.push(stack1.pop());
        }

        size--;
        int ans = stack2.pop();

        for (int i = 0; i < size; i++) {
            stack1.push(stack2.pop());
        }

        return ans;
    }

    /** Get the front element. */
    public int peek() {
        int ans = this.pop();

        // put it back
        this.push(ans);

        return ans;
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return size == 0;
    }
}

References