【一天一道LeetCode】#225. Implement Stack using Queues

一天一道LeetCode

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（一）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)

（二）解题

题目大意：用queue来实现一个stack。

解题思路：queue是先进先出，stack是先进后出。采用一个queue就能实现stack

push：直接压入到queue

pop：需要弹出queue的尾元素，可是queue只能弹出queue的头元素。这时候可以把queue的头元素弹出再压入queue，一直到尾元素弹出即可

top：直接利用queue的back()即可知道stack顶元素

empty：判断queue是否为空。

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {
        que.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        int size = que.size();
        for(int i = 0 ; i < size-1 ;i++){//queue的头元素弹出再压入，直到尾元素弹出
            que.push(que.front());
            que.pop();
        }
        que.pop();
    }

    // Get the top element.
    int top() {
        return que.back();
    }

    // Return whether the stack is empty.
    bool empty() {
        if(que.empty()) return true;
        return false;
    }
private:
    queue<int> que;
};