232. Implement Queue using Stacks

Posted 2020-09-27

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.

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).

class MyQueue {
Stack<Integer> queue = new Stack<Integer>();
// Push element x to the back of queue.
public void push(int x) {
    Stack<Integer> temp = new Stack<Integer>();
    while(!queue.empty()){
        temp.push(queue.pop());
    }
    queue.push(x);
    while(!temp.empty()){
        queue.push(temp.pop());
    }
}

// Removes the element from in front of queue.
public void pop() {
    queue.pop();
}

// Get the front element.
public int peek() {
    return queue.peek();
}

// Return whether the queue is empty.
public boolean empty() {
    return queue.empty();
}