LeetCode 225 Implement Stack using Queues(用佇列來實現棧)(*)

weixin_34162629發表於2017-08-18

翻譯

用佇列來實現棧的例如以下操作。

push(x) —— 將元素x加入進棧 pop() —— 從棧頂移除元素 top() —— 返回棧頂元素 empty() —— 返回棧是否為空 注意: 你必須使用一個僅僅有標準操作的佇列。 也就是說,僅僅有push/pop/size/empty等操作是有效的。

佇列可能不被原生支援。這取決於你所用的語言。 僅僅要你僅僅是用queue的標準操作,你能夠用list或者deque(double-ended queue)來模擬佇列。

你能夠如果全部的操作都是有效的(比如,pop或peek操作不會被用在空棧上)。

原文

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).

分析

對棧和佇列不清楚的話。能夠先看這篇圖文介紹:【演算法】7 分不清棧和佇列?一張圖給你完總體會

至於這道題目。有一道很很相似的題目。

本題是用佇列來實現棧,以下這題是用棧來實現佇列。由於在上一篇中解說過,原理是一樣的,大家能夠自己看看:LeetCode 232 Implement Queue using Stacks(用棧來實現佇列)(*)

程式碼

class Stack {
public:
    queue<int> q, q2;

    // Push element x onto stack.
    void push(int x) {
        q.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        if (q.size() == 1) q.pop();
        else {
            while (q.size() > 1) {
                q2.push(q.front());
                q.pop();
            }
            q.pop();
            while (q2.size() > 0) {
                q.push(q2.front());
                q2.pop();
            }
        }
    }

    // Get the top element.
    int top() {
        if (q.size() == 1) return q.front();
        while (q.size() > 1) {
            q2.push(q.front());
            q.pop();
        }
        int temp = q.front();
        q2.push(q.front());
        q.pop();
        while (q2.size() > 0) {
            q.push(q2.front());
            q2.pop();
        }
        return temp;
    }

    // Return whether the stack is empty.
    bool empty() {
        return q.empty();
    }
};

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