LeetCode 232 Implement Queue using Stacks

Posted 2020-08-06

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：

 1 import java.util.Stack;
 2 
 3 public class MyQueue
 4 {
 5     private Stack<Integer> inputStack;
 6     private Stack<Integer> outputStack;
 7 
 8     public MyQueue()
 9     {
10         inputStack = new Stack<>();
11         outputStack = new Stack<>();
12     }
13 
14     public void push(int x)
15     {
16         while(!outputStack.isEmpty())
17             inputStack.push(outputStack.pop());
18         inputStack.push(x);
19         while(!inputStack.isEmpty())
20             outputStack.push(inputStack.pop());
21     }
22 
23     public void pop()
24     { outputStack.pop(); }
25 
26     public int peek()
27     { return outputStack.peek(); }
28 
29     public boolean empty()
30     { return outputStack.isEmpty(); }
31 }