LeetCode 栈和队列的互换

225. Implement Stack using Queues

栈是先进后出，队列是先进先出，要使用队列实现栈，应该在每次添加元素的时候，将队列整个颠倒，使得删除元素时满足栈的结构。

class MyStack {
    private Queue<Integer>q;
    /** Initialize your data structure here. */
    public MyStack() {
         q = new LinkedList<Integer>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
       q.add(x);
    for(int i = 1; i < q.size(); i ++) { 
        //rotate the queue to make the tail be the head
        q.add(q.poll());
    }  
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
       return q.poll(); 
    }

    /** Get the top element. */
    public int top() {
        return q.peek();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q.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();
 */

232. Implement Queue using Stacks

使用栈实现队列，需要两个栈，互相进行元素的颠倒

class MyQueue {
     Stack<Integer> input;
    Stack<Integer> output;
    /** Initialize your data structure here. */
    public MyQueue() {
      input = new Stack();
      output = new Stack();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        input.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        peek();
        return output.pop();
    }

    /** Get the front element. */
    public int peek() {
         if (output.empty())
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
         return input.empty() && output.empty();
    }
}

/**
 * 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();
 */