LeetCode -- Implement Stacks using Queue

標籤：

Question:

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

 

Analysis:

問題描述：用隊列模仿一個棧。

思路：用兩個隊列模仿一個棧。每次要pop或者peek時，使用隊列倒換一下，剩下最後一個元素單獨處理。若且唯若兩個隊列都為空白時，棧才為空白。

 

Answer：

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.isEmpty()) {                while(q1.size() > 1) {                    int i = q1.poll();                    q2.offer(i);                }                q1.poll();        } else {                while(q2.size() > 1) {                    int i = q2.poll();                    q1.offer(i);                }                q2.poll();        }    }    // Get the top element.    public int top() {        if(!q1.isEmpty()) {            while(q1.size() > 1) {                int i = q1.poll();                q2.offer(i);            }            int i = q1.poll();            q2.offer(i);            return i;        } else {            while(q2.size() > 1) {                int i = q2.poll();                q1.offer(i);            }            int i = q2.poll();            q1.offer(i);            return i;        }    }    // Return whether the stack is empty.    public boolean empty() {        if(q1.size() == 0 && q2.size() == 0)                return true;        return false;    }}

 

LeetCode -- Implement Stacks using Queue