232. 用栈实现队列 (opens new window)

class MyQueue {
    private Stack<Integer> s1,s2;
    public MyQueue() {
        this.s1 = new Stack<>();
        this.s2 = new Stack<>();
    }
    
    public void push(int x) {
        s1.push(x);
    }
    
    public int pop() {
        peek();
        return s2.pop();
    }
    
    public int peek() {
        if(s2.isEmpty()){
            while(!s1.isEmpty())
                s2.push(s1.pop());
        }
        return s2.peek();
    }
    
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

622. 设计循环队列 (opens new window)

方法一：队列

class MyCircularQueue {
    ListNode head;
    ListNode tail;
    int capacity; // 上限
    int size;// 当前容量
    public MyCircularQueue(int k) {
        capacity = k;
        size = 0;
    }
    
    public boolean enQueue(int value) {
        if(isFull())
            return false;
        ListNode node = new ListNode(value);
        if(head==null)
            head = tail = node;
        else{
            tail.next = node;
            tail = node;
        }
        size++;
        return true;
    }
    
    public boolean deQueue() {
        if(isEmpty())
            return false;
        ListNode node = head;
        head = head.next;
        size--;
        return true;
    }
    
    public int Front() {
        if(isEmpty())
            return -1;
        return head.val;
    }
    
    public int Rear() {
        if(isEmpty())
            return -1;
        return tail.val;
    }
    
    public boolean isEmpty() {
        return size == 0;
    }
    
    public boolean isFull() {
        return size == capacity;
    }
}

方法二：数组