LeetCode 232 Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

  Notes:

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

思路：

利用两个栈，每次入队的操作时，将一个栈中的元素全部弹出，后在有元素的栈中压入入队元素，后将该栈的全部元素弹出，压回第一个栈中即可。

解法：

 import java.util.Stack;

 public class MyQueue
 {
     private Stack<Integer> inputStack;
     private Stack<Integer> outputStack;

     public MyQueue()
     {
         inputStack = new Stack<>();
         outputStack = new Stack<>();
     }

     public void push(int x)
     {
         while(!outputStack.isEmpty())
             inputStack.push(outputStack.pop());
         inputStack.push(x);
         while(!inputStack.isEmpty())
             outputStack.push(inputStack.pop());
     }

     public void pop()
     { outputStack.pop(); }

     public int peek()
     { return outputStack.peek(); }

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