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.

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:

We use two stacks to implement the queue.

The new element always get pushed into the input stack.

When we need to peek() or pop(), we check whether the output stack is empty.

If output stack is not empty, we simply take the top element of it.

Otherwise, we put move all the elements from input stack to output stack. Thus these elements are in reverse order.

The amortized time complexity for each operation is O(1).



Code:

public class MyQueue {

    private Stack<Integer> input;
    private 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();
    }
}

/**
 * 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();
 */