lc面试准备:Implement Queue using Stacks

1 题目

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).

接口: 实现4个方法

2 思路

用2个stack，inbox和outbox

Queue:

• Push the new element onto inbox

Dequeue:

• If outbox is empty, refill it by popping each element from inbox and pushing it onto outbox
• Pop and return the top element from outbox

each element will be in each stack exactly once - meaning each element will be pushed twice and popped twice, giving amortized constant time operations.(用这个方法，每个元素只在两个stack中存储一份，每个元素将会被push pop两次，MyQueue的pop操作的时间复杂度是分摊的时间常数，最好O(1)，最坏O(n)).

复杂度:push O(1); pop O(1) or O(n); peek O(1) or O(n); empty O(1)

3 代码

MyQueue.java

import java.util.LinkedList;

// Java编程思想推荐在使用stack的时候，用LinkedList替代。
class MyQueue {
	LinkedList<Integer> inbox = new LinkedList<Integer>();
	LinkedList<Integer> outbox = new LinkedList<Integer>();

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

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

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

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

4 总结

• 巧妙的用两个statck实现queue,数据只存储一份，很好的考察基本的数据结构能力。
• 由于题目假设pop和peek都不会在队列为空的时候执行，避免了Null Pointer Exception.

5 参考

• leetcode
• How to implement a queue using two stacks?