225. Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is emptyoperations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

    class MyStack {
      Queue<Integer> q = new LinkedList<Integer>();
      
      // Push element x onto stack.
      public void push(int x) {
          q.add(x);
          
          int n = q.size();
          while (n-- > 1)
              q.add(q.poll());
      }
  
      // Removes the element on top of the stack.
      public int pop() {
          return q.poll();
      }
  
      // Get the top element.
      public int top() {
          return q.peek();
      }
  
      // Return whether the stack is empty.
      public boolean empty() {
          return q.isEmpty();
      }
  
    }

  用一个queue即可，遇到要push的，先push到队尾，然后一边poll一边offer相当于除队尾外reverse。

• Queue的remove(), poll()都是取出第一个element，用poll()更好因为如果为空可以返回null
• Queue的offer(),add()都是加入新元素，用offer更有效率
• Queue的peek(),element()都是查询第一个element，用peek更好因为如果为空可以返回null

另，stack有empty()和isEmpty()方法，源代码显示其实是一致的，只是后续引入了isEmpty()保留了empty方法