leetcode 225: Implement Stack using Queues

Implement Stack using Queues

Total Accepted: 3468
Total Submissions: 11988

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).

Update (2015-06-11):

The class name of the Java function had been updated to
MyStack instead of Stack.

[思路]

没想出啥好的办法. 就是两个queues, 轮流把除最后一个之外的都放入另一个queue中.

[CODE]

class MyStack {
// Push element x onto stack.
Queue<Integer> q1 = new LinkedList<Integer>();
Queue<Integer> q2 = new LinkedList<Integer>();

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> q = q1;
q1 = q2;
q2 = q;
}

// Get the top element.
public int top() {
while(q1.size()>1) q2.offer(q1.poll());
int x = q1.poll();
q2.offer(x);
Queue<Integer> q = q1;
q1 = q2;
q2 = q;
return x;
}

// Return whether the stack is empty.
public boolean empty() {
return q1.isEmpty();
}
}