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p225

admin 11月 12, 2020 0

LeetCode p225 Implement Stack using Queues 题解

1.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 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).

题意：

用队列实现栈

2.解题思路:

两个队列倒来倒去

3.代码

[title] [] [url] [link text]

 
public class  {

    private Queue<Integer> queue1;
	private Queue<Integer> queue2;

	
	public () {
		queue1 = new LinkedList<Integer>();
		queue2 = new LinkedList<Integer>();
	}

	/** Push element x onto stack. */
	public void push(int x) {
		if (queue1.size() == 0) {
			queue1.add(x);
			while (queue2.size() > 0) {
				queue1.add(queue2.poll());
			}
		} else {
			queue2.add(x);
			while (queue1.size() > 0) {
				queue2.add(queue1.poll());
			}
		}

	}

	/** Removes the element on top of the stack and returns that element. */
	public int pop() {
		if (queue1.size() == 0) {
			return queue2.poll();
		} else {
			return queue1.poll();
		}
	}

	/** Get the top element. */
	public int top() {
		if (queue1.size() == 0) {
			return queue2.peek();
		} else {
			return queue1.peek();
		}
	}

	/** Returns whether the stack is empty. */
	public boolean empty() {

		return queue1.isEmpty() && queue2.isEmpty();
	}
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */

4.一些总结：