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前言

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两种数据结构底层其实都是数组或者链表实现的，只是 API 限定了它们的特性，那么今天就来看看如何使用「栈」的特性来实现一个「队列」，如何用「队列」实现一个「栈」

一、力扣225. 用队列实现栈

class MyStack {Deque<Integer> deq1;Deque<Integer> deq2;public MyStack() {deq1 = new LinkedList<>();deq2 = new LinkedList<>();}public void push(int x) {deq1.offerLast(x);}public int pop() {while(deq1.size()>1){deq2.offerLast(deq1.pollFirst());}int res = deq1.pollFirst();while(!deq2.isEmpty()){deq1.offerLast(deq2.pollFirst());}return res;}public int top() {while(deq1.size() > 1){deq2.offerLast(deq1.pollFirst());}int res = deq1.pollFirst();deq2.offerLast(res);while(!deq2.isEmpty()){deq1.offerLast(deq2.pollFirst());}return res;}public boolean empty() {return deq1.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();*/

二、力扣232. 用栈实现队列

class MyQueue {Deque<Integer> deq1;Deque<Integer> deq2;public MyQueue() {deq1 = new LinkedList<>();deq2 = new LinkedList<>();}public void push(int x) {deq1.offerLast(x);}public int pop() {while(deq1.size() > 1){deq2.offerLast(deq1.pollLast());}int res = deq1.pollLast();while(!deq2.isEmpty()){deq1.offerLast(deq2.pollLast());}return res;}public int peek() {while(deq1.size() > 1){deq2.offerLast(deq1.pollLast());}int res = deq1.pollLast();deq2.offerLast(res);while(!deq2.isEmpty()){deq1.offerLast(deq2.pollLast());}return res;}public boolean empty() {return deq1.isEmpty();}
}/*** Your MyQueue object will be instantiated and called as such:* MyQueue obj = new MyQueue();* obj.push(x);* int param_2 = obj.pop();* int param_3 = obj.peek();* boolean param_4 = obj.empty();*/