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
, andis 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;
}
}