【leetcode】第五章 栈与队列part01

232.用栈实现队列

• 用两个栈来实现
• 当pop时，检测out栈是否为空，若为空，则将in栈的元素全都放入out栈中

class MyQueue {

    private Stack<Integer> in;
    private Stack<Integer> out;

    public MyQueue() {
        in = new Stack<>();
        out = new Stack<>();
    }

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

    public int pop() {

        if (out.isEmpty()) {
            while (!in.isEmpty()) {
                out.push(in.pop());
            }
        }
        return out.pop();
    }

    public int peek() {
        if (out.isEmpty()) {
            while (!in.isEmpty()) {
                out.push(in.pop());
            }
        }
        return out.peek();
    }

    public boolean empty() {
        return in.isEmpty() && out.isEmpty();
    }
}

225. 用队列实现栈

• 单个队列方法
• 将前面n-1个元素弹出并加入底部，这样就可以模拟后进先出

class MyStack {
    // 用单个队列
    // 将前面n-1个元素弹出并加入底部
    private Queue<Integer> a;

    public MyStack() {
        a = new LinkedList<>();
    }
    
    public void push(int x) {
        // size-1
        int n = a.size();
        a.offer(x);
        while (n > 0) {
            a.offer(a.poll());
            n--;
        }
    }
    
    public int pop() {
        return a.poll();
    }
    
    public int top() {
        return a.peek();
    }
    
    public boolean empty() {
        return a.isEmpty();
    }
}