[LeetCode] 232. 225 Implement Queue/Stack using Stacks/Queues

Problem 232. 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).

Solution 232. Implement Queue using Stacks

class MyQueue {

    Stack<Integer> input;
    Stack<Integer> output;
    /** Initialize your data structure here. */
    public MyQueue() {
        input = new Stack<>();
        output = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        input.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        return output.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        if (output.isEmpty()) {
            while (!input.isEmpty()) output.push(input.pop());
        }
        return output.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return input.isEmpty() && output.isEmpty();
    }
}

Problem 225. Implement Stack using Queues

Implement the following operations of a stack using queues.

push(x) — Push element x onto stack.
pop() — Removes the element on top of the stack.
top() — Get the top element.
empty() — Return whether the stack is empty.
Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

You must use only standard operations of a queue — which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Solution 225. Implement Stack using Queues

Using one queue

class MyStack {

    Queue<Integer> q;
    /** Initialize your data structure here. */
    public MyStack() {
        q = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        q.offer(x);
        for (int i = 1; i < q.size(); i++) q.offer(q.poll());
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return q.isEmpty() ? -1 : q.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return q.isEmpty() ? -1 : q.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q.isEmpty();
    }
}

Using two queues

class MyStack {

    Queue<Integer> q1;
    Queue<Integer> q2;
    /** Initialize your data structure here. */
    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        if (q1.isEmpty()) {
            q1.offer(x);
            for (int i = 0; i < q2.size(); i++) q1.offer(q2.poll());
        } else {
            q2.offer(x);
            for (int i = 0; i < q1.size(); i++) q2.offer(q1.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if (q1.isEmpty()) return q2.poll();
        else return q1.poll();
    }
    
    /** Get the top element. */
    public int top() {
        if (q1.isEmpty() && q2.isEmpty()) return -1;
        else return q1.isEmpty() ? q2.peek() : q1.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q1.isEmpty() && q2.isEmpty();
    }
}