原题例如以下：
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 backpeek/pop from front,size,
andis emptyoperations 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).
基本思路是：如果有两个队列Q1和Q2，当二者都为空时。入栈操作能够用入队操作来模拟，能够随便选一个空队列，如果选Q1进行入栈操作。如今如果a,b,c依次入栈了（即依次进入队列Q1）。这时如果想模拟出栈操作，则须要将c出栈。由于在栈顶。这时候能够考虑用空队列Q2，将a，b依次从Q1中出队，而后进入队列Q2，将Q1的最后一个元素c出队就可以。此时Q1变为了空队列。Q2中有两个元素，队头元素为a。队尾元素为b。接下来如果再运行入栈操作，则须要将元素进入到Q1和Q2中的非空队列，即进入Q2队列，出栈的话，就跟前面的一样。将Q2除最后一个元素外所有出队，并依次进入队列Q1，再将Q2的最后一个元素出队就可以。
Java实现代码例如以下：
class MyStack { LinkedList<Integer> queue1 = new LinkedList<Integer>();
LinkedList<Integer> queue2 = new LinkedList<Integer>();
// Push element x onto stack.
public void push(int x) {
if(queue1.size()==0&&queue2.size()==0)
{
queue1.offer(x);
}
else if(queue1.size()==0)
{
queue2.offer(x);
}
else
{
queue1.offer(x);
}
}
// Removes the element on top of the stack.
public void pop() {
if(queue1.size()!= 0)
{
int length = queue1.size();
for(int i =0;i<length-1;i++)
{
queue2.offer(queue1.poll());
}
queue1.poll();
}
else
{
int length = queue2.size();
for(int i =0;i<length-1;i++)
{
queue1.offer(queue2.poll());
}
queue2.poll();
}
}
// Get the top element.
public int top() {
if(queue1.size()!= 0)
{
int length = queue1.size();
for(int i =0;i<length-1;i++)
{
queue2.offer(queue1.poll());
}
int result = queue1.element();
queue2.offer(queue1.poll());
return result;
}
else
{
int length = queue2.size();
for(int i =0;i<length-1;i++)
{
queue1.offer(queue2.poll());
}
int result = queue2.element();
queue1.offer(queue2.poll());
return result;
}
}
// Return whether the stack is empty.
public boolean empty() {
return queue1.size()==0&&queue2.size()==0;
}
}