Implement Stack using Queues

Leetcode

題意:

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).

解題思路:

一旦插入新值時,要把前面的所有值不斷poll offer到queue的尾端。

class MyStack {
    // Push element x onto stack.
    Queue<Integer> q = new LinkedList<Integer>();
    public void push(int x) {
        q.offer(x);
        for (int i = 0; i < q.size() - 1; i++) {
            q.offer(q.poll());
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        q.poll();
    }

    // Get the top element.
    public int top() {
        return q.peek();
    }

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

Reference

  1. https://leetcode.com/discuss/46975/a-simple-c-solution

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