Leetcode 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.

Notes:

You must use only standard operations of a queue -- which means only

push to back

,

peek/pop from front

,

size

, and

is empty

operations 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).

解题思路：

只要实现对了push函数，后面三个直接调用队列的函数即可。这种方法的原理就是每次把新加入的数插到前头，这样队列保存的顺序和栈的顺序是相反的，它们的取出方式也是反的，那么反反得正，就是我们需要的顺序了。我们需要一个辅助队列tmp，把s的元素也逆着顺序存入tmp中，此时加入新元素x，再把tmp中的元素存回来，这样就是我们要的顺序了，其他三个操作也就直接调用队列的操作即可。

Java Code:

import java.util.LinkedList;
import java.util.Queue;

public class ImStackUsingQue {
class MyStack {
private Queue<Integer> q = new LinkedList<Integer>();
// Push element x onto stack.
public void push(int x) {
Queue<Integer> temp = new LinkedList<Integer>();
while(!q.isEmpty()) {
temp.add(q.peek());
q.remove();
}
q.add(x);
while(!temp.isEmpty()) {
q.add(temp.peek());
temp.remove();
}
}

// Removes the element on top of the stack.
public void pop() {
q.remove();
}

// Get the top element.
public int top() {
return q.peek();
}

// Return whether the stack is empty.
public boolean empty() {
return q.isEmpty();
}
}
}

Reference:

1. http://www.cnblogs.com/grandyang/p/4568796.html