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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.


use two queues

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() {
        if(q1.size() == 0) return;
        while(q1.size() > 1){
            q2.offer(q1.poll());
        }
        q1.poll();
        Queue<Integer> q = q2;
        q2 = q1;
        q1 = q;
    }

    // Get the top element.
    public int top() {
        if(q1.size() == 0) return 0;
        while(q1.size() > 1){
            q2.offer(q1.poll());
        }
        int top = q1.peek();
        q2.offer(q1.poll());
        Queue<Integer> q = q2;
        q2 = q1;
        q1 = q;
        return top;
    }

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


本文标签： LeetCodeimplementqueuesStack

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