LC.P232[用栈实现队列]

方法一：模拟

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class MyQueue {

    Deque<Integer> s1;
    Deque<Integer> s2;

    public MyQueue() {
        s1 = new ArrayDeque<>();
        s2 = new ArrayDeque<>();
    }
    
    public void push(int x) {
        s1.push(x);
    }
    
    public int pop() {
        int x = peek();
        s2.pop();
        return x;
    }
    
    public int peek() {
        if (!s2.isEmpty()) return s2.peek();
        if (s1.isEmpty()) return -1;
        while (!s1.isEmpty()) {
            s2.push(s1.pop());
        }
        return s2.peek();
    }
    
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

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