题目描述

• 感觉栈实现队列更简单，不过这个也还好
• 写法有点像 JVM GC 里的复制算法
  

思路 & 代码

• 两个队列实现栈：from、to
• from：实际上的栈，存储元素就是按照栈的顺序来，负责pop、top
• to：辅助队列，永远为空 （是不是很像幸存区的 from & to 呢 ^ ^）
• MyStack.size() == from.size()，因为 to 为空队列，只是函数过程可能暂存一下。
• push 要特别注意，直接 push 到 to 里，然后把 from 的元素逐个 push 到 to 中，此时 to 为实际上的栈，然后再 from & to 互换引用。

/*** 思路：使用辅助队列，每次push都替一下，保证实际上 queue1 是按照栈顺序来的* 还是有点类似 JVM GC 的复制算法
*/
class MyStack {LinkedList<Integer> from;LinkedList<Integer> to;/** Initialize your data structure here. */public MyStack() {from = new LinkedList<>();to = new LinkedList<>();}// add：相当于 Queue.push()// removeFirst: 相当于 Queue.pop()/** Push element x onto stack. */public void push(int x) {to.add(x);while(!from.isEmpty()){to.add(from.removeFirst());}LinkedList<Integer> temp = from;from = to;to = temp;}/** Removes the element on top of the stack and returns that element. */public int pop() {return from.removeFirst();}/** Get the top element. */public int top() {// 相当于 Queue.peek()return from.get(0);}/** Returns whether the stack is empty. */public boolean empty() {return from.isEmpty();}
}/*** Your MyStack object will be instantiated and called as such:* MyStack obj = new MyStack();* obj.push(x);* int param_2 = obj.pop();* int param_3 = obj.top();* boolean param_4 = obj.empty();*/

更新版

• 换成 ArrayDeque

class MyStack {Deque<Integer> in;Deque<Integer> out;public MyStack() {in = new ArrayDeque<>();out = new ArrayDeque<>();}public void push(int x) {in.offer(x);while(!out.isEmpty()) {in.offer(out.poll());}Deque<Integer> temp = in;in = out;out = temp;}public int pop() {return out.pop();}public int top() {return out.element();}public boolean empty() {return out.isEmpty();}
}