225. Implement Stack using Queues

难度: Easy

题目描述

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

 

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

 

Follow-up: Can you implement the stack using only one queue?

解题思路

代码实现

解决方案

java

class MyStack {

    private Queue<Integer> q1;
    private Queue<Integer> q2;
    private int top;

    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
    }

    public void push(int x) {
        q1.offer(x);
        top = x;
    }

    public int pop() {
        if (q1.isEmpty()) {
            top = 0;
            return 0;
        }
        while (q1.size() > 1) {
            if (q1.size() == 2) {
                top = q1.peek();
            }
            q2.offer(q1.poll());
        }
        int res = q1.poll();
        while (q2.size() > 0) {
            q1.offer(q2.poll());
        }
        if (q1.size() == 0) {
            top = 0;
        }
        return res;
    }

    public int top() {

        return top;
    }

    public boolean empty() {

        return q1.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();
 */