LeetCode-Implement Queue using Stacks

Posted 2020-08-09 LiBlog

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).

Analysis:

Maintain a push stack and a pop stack. For pop(), if pop stack is empty, we move all elements in push stack into pop stack. After that, we can always pop until pop stack is empty.

Solution:

class MyQueue {
    Stack<Integer> pushQ = new Stack<Integer>();
    Stack<Integer> popQ = new Stack<Integer>();
    
    // Push element x to the back of queue.
    public void push(int x) {
        pushQ.push(x);
    }

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

    // Get the front element.
    public int peek() {
        if (popQ.isEmpty())
            while (!pushQ.empty()){
                popQ.push(pushQ.pop());
            }
        return popQ.peek();
    }

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