lc面试准备:Implement Queue using Stacks

1 题目

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?