LeetCode - 225 用队列实现栈

使用两个队列

class MyStack {
    
    //使用两个队列实现栈
    //主队列
    private Deque<Integer> deque1;
    //辅助队列
    private Deque<Integer> deque2;
    public MyStack() {
    
         deque1 = new LinkedList<>();
         deque2 = new LinkedList<>();
    }
    
    public void push(int x) {
    
        //首先把主队列的所有值放入辅助队列中
         while(deque1.size() != 0){
    
             deque2.offerLast(deque1.pollFirst());
         }
         //将x入主队列
         deque1.offerLast(x);
        //将辅助队列的值重新放入主队列中
         while(deque2.size() != 0){
    
             deque1.offerLast(deque2.pollFirst());
         }
         //此时后入队列的x已经位于队头
    }
    
    public int pop() {
    
       return deque1.pollFirst();
    }
    
    public int top() {
    
       return deque1.peekFirst();
    }
    
    public boolean empty() {
    
        return deque1.size() == 0;
    }
}

使用一个队列

class MyStack {
    
    //使用一个队列实现栈
    private Deque<Integer> deque;
    public MyStack() {
    
         deque = new LinkedList<>();
    }
    
    public void push(int x) {
    
        //队列中当前有多少个值
        int qsize = deque.size();
        //将x入队列
        deque.offerLast(x);
        //将队列中前qsize个值放入队尾
        while(qsize > 0){
    
            deque.offerLast(deque.pollFirst());
            qsize--;
        }
         //此时后入队列的x已经位于队头
    }
    
    public int pop() {
    
       return deque.pollFirst();
    }
    
    public int top() {
    
       return deque.peekFirst();
    }
    
    public boolean empty() {
    
        return deque.size() == 0;
    }
}