LeetCode——Implement Stack using Queues

Description：

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).

用队列来实现一个栈。

思路：用两个队列来模拟栈的功能，当前队列(cur)是用来存放数据的，另一个队列是用来替换当前队列的，这样就能操作队尾元素了。两个队列来回切换，一个用来保存数据，另一个用来操作队尾元素。

PS.Java里不能用泛型数组啊，只好用线性表来代替了。代码看起来又复杂了一些。要不要用C++和Python再来一遍。

class MyStack {
public List<Queue<Integer>> queue;
public int cur;
public MyStack() {
queue = new ArrayList<Queue<Integer>>();
queue.add(new LinkedList<Integer>());
queue.add(new LinkedList<Integer>());
cur = 0;
}
// Push element x onto stack.
public void push(int x) {
queue.get(cur).offer(x);
}

// Removes the element on top of the stack.
public void pop() {
while(queue.get(cur).size() > 1) {
queue.get(1-cur).offer(queue.get(cur).poll());
}
queue.get(cur).poll();
cur = 1 - cur;
}

// Get the top element.
public int top() {
while(queue.get(cur).size() > 1) {
queue.get(1-cur).offer(queue.get(cur).poll());
}
int t = queue.get(cur).poll();
queue.get(1-cur).offer(t);
cur = 1 - cur;
return t;
}

// Return whether the stack is empty.
public boolean empty() {
if(queue.get(cur).isEmpty())
return true;
else
return false;
}
}