Brooke

一、題目

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

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

 

Example 1:

Input ["MyQueue", "push", "push", "peek", "pop", "empty"] [[], [1], [2], [], [], []] Output [null, null, null, 1, 1, false] Explanation MyQueue myQueue = new MyQueue(); myQueue.push(1); // queue is: [1] myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue) myQueue.peek(); // return 1 myQueue.pop(); // return 1, queue is [2] myQueue.empty(); // return false

 

Constraints:

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

 

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

 

二、程式作法

public class MyQueue
{
    private Stack<int> set = new Stack<int>();
    private bool dirUp = false;
    public MyQueue()
    {

    }

    public void Push(int x)
    {
        if (dirUp == false)
            set.Push(x);
        else
        {
            Stack<int> temp = new Stack<int>();
            while (set.Count > 0)
            {
                temp.Push(set.Pop());
            }
            set = temp;
            set.Push(x);
            dirUp = false;
        }
    }

    public int Pop()
    {
        if (dirUp == true)
            return set.Pop();
        else
        {
            Stack<int> temp = new Stack<int>();
            while (set.Count > 0)
            {
                temp.Push(set.Pop());
            }
            set = temp;
            dirUp = true;
            return set.Pop();
        }
    }

    public int Peek()
    {
        if (dirUp == true)
            return set.Peek();
        else
        {
            Stack<int> temp = new Stack<int>();
            while (set.Count > 0)
            {
                temp.Push(set.Pop());
            }
            set = temp;
            dirUp = true;
            return set.Peek();
        }
    }

    public bool Empty()
    {
        return (set.Count == 0) ? true : false;
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.Push(x);
 * int param_2 = obj.Pop();
 * int param_3 = obj.Peek();
 * bool param_4 = obj.Empty();
 */

 

三、思路筆記

題目要的是用兩個 stack 去實作 queue。