[leetcode] Implement Queue using Stacks

最新推荐文章于 2021-11-08 15:11:52 发布

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本文详细阐述了如何利用两个栈来模拟队列的行为，包括入队、出队、获取队首元素和判断队列是否为空的操作。通过实例演示，帮助读者理解栈与队列之间的转换与应用。

from : https://leetcode.com/problems/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).

class MyQueue {
	Stack<Integer> psh = new Stack<Integer>();
	Stack<Integer> pap = new Stack<Integer>();

	// Push element x to the back of queue.
	public void push(int x) {
		psh.push(new Integer(x));
	}

	// Removes the element from in front of queue.
	public void pop() {
		if (!pap.isEmpty()) {
			pap.pop();
		} else {
			while (!psh.isEmpty()) {
				pap.push(psh.pop());
			}
			pap.pop();
		}
	}

	// Get the front element.
	public int peek() {
		if (!pap.isEmpty()) {
			return pap.peek();
		} else {
			while (!psh.isEmpty()) {
				pap.push(psh.pop());
			}
			return pap.peek();
		}
	}

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

}