Implement Queue using Stacks
来源:互联网 发布:gogs windows 编辑:程序博客网 时间:2024/05/02 16:23
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).
队列:先进先出。push在队尾,pop在队首,peek在队首。
栈:后进先出。只能对栈顶进行操作。
两种数据结构在逻辑上正好相反,反反得正,所以需要两个栈进行顺序的颠倒。最简单的方法是以栈1作为存储空间,然后每次需要颠倒的时候(pop和peek),都去栈2中颠倒一下然后还原回栈1。毫无疑问,这是一种很烂的算法,因为无论是从时间上,还是空间上,都造成了极大的浪费。我们完全可以把栈1用作入队操作,把栈2用作出队操作。
class MyQueue { private Stack<Integer> s1; private Stack<Integer> s2; public MyQueue() { // TODO Auto-generated constructor stub s1 = new Stack<Integer>(); s2 = new Stack<Integer>(); } // Push element x to the back of queue. public void push(int x) { s1.push(x); } // Removes the element from in front of queue. public void pop() { if(!s2.empty()){ s2.pop(); } else { while(!s1.empty()){ s2.push(s1.pop()); } s2.pop(); } } // Get the front element. public int peek() { if(!s2.isEmpty()){ return s2.peek(); } else { while(!s1.empty()){ s2.push(s1.pop()); } return s2.peek(); } } // Return whether the queue is empty. public boolean empty() { return s1.isEmpty() && s2.isEmpty(); }}
简单介绍一下原理:
这里,我把【栈1 pop 一个数,栈2 push 这个数】操作简称为【颠倒】。如果进队顺序是1、2、3、4,那么期望的出队顺序也是1、2、3、4。
- 操作1:往栈1 push 1、2;颠倒1、2到栈2;push 3、4,颠倒3、4到栈2。栈2从顶部到底部的顺序是1、2、3、4。
- 操作2:往栈1 push 1、2、3;颠倒1、2、3到栈2; push 4,颠倒4到栈2。栈2从顶部到底部的顺序是1、2、3、4。
不难发现出队操作就是栈2的不断pop,无论操作1,还是操作2,无论push和颠倒的顺序和时机,对出队顺序没有影响。
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Implement Queue using Stacks
- Android原生页面同Html5的交互
- 还是有希望的
- GetLastError 错误码全解析
- 54.EasyArcLoading
- android 优秀资料转载目录
- Implement Queue using Stacks
- 使用Fragment仿微信之二Fragemnt使用及overflow菜单使用
- PHP 单引号双引号的区别
- iOS开发代码编写规范
- BZOJ 1877
- HBase总结(6)--比较过滤器
- Thinkphp项目结构
- Python对字母字符(串)中大小写转换函数--upper() 和 lower()
- Unity3D -- 设置Android支付横屏竖屏