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).
用两个队列来实现堆栈后进先出的功能。代码如下:
class MyStack { Queue<Integer> q1 = new LinkedList<Integer>(); Queue<Integer> q2 = new LinkedList<Integer>(); // 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<Integer> tem = q1; q1 = q2; q2 = tem; } // Get the top element. public int top() { while(q1.size() > 1) q2.offer(q1.poll()); int result = q1.poll(); q2.offer(result); Queue<Integer> tem = q1; q1 = q2; q2 = tem; return result; } // Return whether the stack is empty. public boolean empty() { return q1.isEmpty(); } }