LC.P225[用队列实现栈]

方法一：模拟

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import java.util.Deque;

class MyStack {
    Deque<Integer> q1;
    Deque<Integer> q2;

    public MyStack() {
        q1 = new ArrayDeque<>();
        q2 = new ArrayDeque<>();
    }

    public void push(int x) {
        q2.offer(x);
        while (!q1.isEmpty()) {
            q2.offer(q1.poll());
        }
        Deque<Integer> q = q1;
        q1 = q2;
        q2 = q;
    }

    public int pop() {
        return q1.poll();
    }

    public int top() {
        return q1.peek();
    }

    public boolean empty() {
        return q1.isEmpty();
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$