Leetcode_232_Implement Queue using Stacks

本文是在学习中的总结，欢迎转载但请注明出处：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）题意为用栈来实现队列。

（2）要用栈来实现队列，首先需要了解栈和队列的性质。栈：先进后出，只能在栈顶增加和删除元素；队列：先进先出，只能在队尾增加元素，从队头删除元素。这样，用栈实现队列，就需要对两个栈进行操作，这里需要指定其中一个栈为存储元素的栈，假定为stack2，另一个为stack1。当有元素加入时，首先判断stack2是否为空（可以认为stack2是目标队列存放元素的实体），如果不为空，则需要将stack2中的元素全部放入(辅助栈)stack1中，这样stack1中存储的第一个元素为队尾元素；然后，将待加入队列的元素加入到stack1中，这样相当于实现了将入队的元素放入队尾；最后，将stack1中的元素全部放入stack2中，这样stack2的栈顶元素就变为队列第一个元素，对队列的pop和peek的操作就可以直接通过对stack2进行操作即可。

（3）详情见下方代码。希望本文对你有所帮助。

算法代码实现如下：

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();
}
}