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题目：

用两个栈实现队列

实现

//coding = javapublic class SQueue {    private Stack<Integer> stack1 = new Stack();    private Stack<Integer> stack2 = new Stack();    public void add(int data){        stack1.add(data);    }    public int pop(){        if(stack2.empty()){            while (!stack1.empty()){                stack2.add(stack1.pop());            }        }        if(stack2.empty()){            throw new RuntimeException("队列为空");        }        int result = stack2.pop();        return result;    }    public static void main(String[] args) throws Exception {        SQueue queue = new SQueue();        //入队操作        for (int i = 0; i < 5; i++) {            queue.add(i);        }        //出队操作        System.out.println(queue.pop());        System.out.println(queue.pop());        System.out.println(queue.pop());    }}

用链表实现队列

// coding = javapublic class lQueue {    Node head;    Node current;    public void add(int data){        if(head==null){            head = new Node(data);            current=head;        }        else{            current.next=new Node(data);            current=current.next;        }    }    public  int pop(){        if(current==null){            throw new RuntimeException("队列为空");        }        Node result=head;        head=head.next;        return result.value;    }    public static void main(String[] args) throws Exception {        lQueue queue = new lQueue();        //入队操作        for (int i = 0; i < 5; i++) {            queue.add(i);        }        //出队操作        System.out.println(queue.pop());        System.out.println(queue.pop());        System.out.println(queue.pop());    }}

用两个队列实现栈

//coding = javapublic class QStack {    private Queue<Integer> queue1=new ArrayDeque();    private Queue<Integer> queue2=new ArrayDeque();    public void push(int data){        if(queue1.isEmpty() && queue2.isEmpty()){            queue1.add(data);        }        else if(queue1.isEmpty() && !queue2.isEmpty()){            queue2.add(data);        }       else if(!queue1.isEmpty() && queue2.isEmpty()){            queue1.add(data);        }    }    public int pop(){        if(queue1.isEmpty() && queue2.isEmpty()){            throw new RuntimeException("栈为空");        }        int result = 0;        if(queue1.isEmpty() && !queue2.isEmpty()) {            while (!queue2.isEmpty()) {                result = queue2.poll();                if (!queue2.isEmpty()) {                    queue1.add(result);                }            }        }        else if (!queue1.isEmpty() && queue2.isEmpty()) {                while (!queue1.isEmpty()) {                    result = queue1.poll();                    if (!queue1.isEmpty()) {                        queue2.add(result);                    }                }            }        return result;        }    public static void main(String[] args) throws Exception {        QStack stack = new QStack();        stack.push(1);        stack.push(2);        stack.push(3);        System.out.println(stack.pop());        System.out.println(stack.pop());        stack.push(4);    }}

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