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题目

• 原题连接：232. 用栈实现队列

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1- 思路

思路

• ① 用两个栈来实现队列，一个 in 入栈 和一个 out 出栈
• ② push 入队：入栈逻辑：即将元素加入到 in 栈 里即可
• ③ pop 出队：出栈逻辑，先将 in栈 内容 依次弹出，加入到 out栈 中
• ④ peek 取顶：先 pop 再 push，返回弹出的元素

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2- 实现

⭐232. 用栈实现队列——题解思路

class MyQueue {


    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }
    
    public void push(int x) {
        stackIn.push(x);
    }
    
    public int pop() {
        dumpStackIn();
        return stackOut.pop();
    }
    
    public int peek() {
        dumpStackIn();
        return stackOut.peek();
    }
    
    public boolean empty() {
        return stackIn.isEmpty() && stackOut.isEmpty();
    }


    public void dumpStackIn(){
        if(!stackOut.isEmpty()) return;
        while(!stackIn.isEmpty()){
            stackOut.push(stackIn.pop());
        }
    }
}

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3- ACM 实现

public class MyQueue {

    Stack<Integer> stackIn ;
    Stack<Integer> stackOut ;

    public MyQueue(){
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }

    public void push(int x){
        stackIn.push(x);
    }

    public int pop(){
        dumpIn();
        return stackOut.pop();
    }

    public int peek(){
        dumpIn();
        return stackOut.peek();
    }

    public boolean empty(){
        return stackIn.isEmpty()&&stackOut.isEmpty();
    }

    public void dumpIn(){
        if(!stackOut.isEmpty()) return;
        while(!stackIn.isEmpty()){
            stackOut.push(stackIn.pop());
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        MyQueue queue = new MyQueue();

        while (sc.hasNext()) {
            String command = sc.next();
            switch (command) {
                case "MyQueue":
                    queue = new MyQueue();
                case "push":
                    if (sc.hasNextInt()) {
                        int x = sc.nextInt();
                        queue.push(x);
                    }
                    break;
                case "pop":
                    System.out.println(queue.pop());
                    break;
                case "peek":
                    System.out.println(queue.peek());
                    break;
                case "empty":
                    System.out.println(queue.empty());
                    break;
            }
        }
        sc.close();
    }

}

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