【题目】
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 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).
【解法一:用两个队列,push: O(1),pop: O(n),top: O(n)】
用两个队列q1,q2实现一个栈。push时把新元素添加到q1的队尾。pop时把q1中除最后一个元素外逐个添加到q2中,然后pop掉q1中的最后一个元素,然后注意记得q1和q2,以保证我们添加元素时始终向q1中添加。top的道理类似。
class MyStack { private Queue<Integer> q1 = new LinkedList<>(); private Queue<Integer> q2 = new LinkedList<>(); // Push element x onto stack. public void push(int x) { q1.offer(x); } // Removes the element on top of the stack. public void pop() { while (q1.size() > 1) { q2.offer(q1.poll()); } q1.poll(); Queue tmp = q1; q1 = q2; q2 = tmp; } // Get the top element. public int top() { while (q1.size() > 1) { q2.offer(q1.poll()); } int top = q1.peek(); q2.offer(q1.poll()); Queue tmp = q1; q1 = q2; q2 = tmp; return top; } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); } }
【解法二:用两个队列,push: O(n),pop: O(1),top: O(1)】
所有元素都倒序保存在q1中,即后添加的元素在q1的最前端,如何做到呢?每次push时,把新元素放到空的q2,然后把q1中元素逐个添加到q2的队尾,最后交换q1和q2。这样q1队首的元素就是最后添加的元素,pop和top直接返回q1队首的元素就好。
class MyStack { private Queue<Integer> q1 = new LinkedList<>(); private Queue<Integer> q2 = new LinkedList<>(); // Push element x onto stack. public void push(int x) { q2.offer(x); while (!q1.isEmpty()) { q2.offer(q1.poll()); } Queue tmp = q1; q1 = q2; q2 = tmp; } // Removes the element on top of the stack. public void pop() { q1.poll(); } // Get the top element. public int top() { return q1.peek(); } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); } }
【解法三:一个队列,push: O(1),pop: O(n),top: O(n)】
push时直接添加到队尾就好。pop和top时,把队列除最后一个元素外,逐个循环添加到队列的尾部。
class MyStack { private Queue<Integer> q = new LinkedList<>(); // Push element x onto stack. public void push(int x) { q.offer(x); } // Removes the element on top of the stack. public void pop() { int size = q.size(); for (int i = 1; i < size; i++) { q.offer(q.poll()); } q.poll(); } // Get the top element. public int top() { int size = q.size(); for (int i = 1; i < size; i++) { q.offer(q.poll()); } int top = q.peek(); q.offer(q.poll()); return top; } // Return whether the stack is empty. public boolean empty() { return q.isEmpty(); } }
其实三种实现,大同小异,无非是队列中元素的不停pop与push以得到最后一个元素。
时间: 2024-10-24 03:01:59