Lettcode_232_Implement Queue using Stacks，lettcode

Lettcode_232_Implement Queue using Stacks，lettcode

本文是在學習中的總結，歡迎轉載但請註明出處：http://blog.csdn.net/pistolove/article/details/48392363

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：

（1）題意為用棧來實現隊列。

（2）要用棧來實現隊列，首先需要瞭解棧和隊列的性質。棧：先進後出，只能在棧頂增加和刪除元素；隊列：先進先出，只能在隊尾增加元素，從隊頭刪除元素。這樣，用棧實現隊列，就需要對兩個棧進行操作，這裡需要指定其中一個棧為儲存元素的棧，假定為stack2，另一個為stack1。當有元素加入時，首先判斷stack2是否為空白（可以認為stack2是目標隊列存放元素的實體），如果不為空白，則需要將stack2中的元素全部放入(輔助棧)stack1中，這樣stack1中儲存的第一個元素為隊尾元素；然後，將待排入佇列的元素加入到stack1中，這樣相當於實現了將入隊的元素放入隊尾；最後，將stack1中的元素全部放入stack2中，這樣stack2的棧頂元素就變為隊列第一個元素，對隊列的pop和peek的操作就可以直接通過對stack2進行操作即可。

（3）詳情見下方代碼。希望本文對你有所協助。

演算法代碼實現如下：

package leetcode;import java.util.Stack;/** * @author liqqc * */public class Implement_Queue_using_Stacks {public Stack<Integer> _stack1 = new Stack<Integer>();public Stack<Integer> _stack2 = new Stack<Integer>();public void push(int x) {while (!_stack2.isEmpty()) {_stack1.push(_stack2.pop());}_stack1.push(x);while (!_stack1.isEmpty()) {_stack2.push(_stack1.pop());}}// Removes the element from in front of queue.public void pop() {_stack2.pop();}// Get the front element.public int peek() {return _stack2.peek();}// Return whether the queue is empty.public boolean empty() {return _stack2.isEmpty();}}

著作權聲明：本文為博主原創文章，未經博主允許不得轉載。