Implement Queue using Stacks-蒲公英云

Implement Queue using Stacks

红太狼 2022-08-03 08:36 35阅读 0赞

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).

思路：额外数组记录状态，注意peek是查看最近一次加入的元素

class MyQueue {
    // Push element x to the back of queue.
    Stack<Integer> stack=new Stack<>();
    List<Integer> list=new ArrayList<>();
    public void push(int x) {
        stack.push(x);
        list.add(x);
    }
    // Removes the element from in front of queue.
    public void pop() {
        stack.remove(list.get(0));
        list.remove(0);
    }
    // Get the front element.
    public int peek() {
        return list.get(0);
    }
    // Return whether the queue is empty.
    public boolean empty() {
        return stack.isEmpty();
    }
}