[每日 LeetCode] 225. Implement Stack using Queues

原文链接 [每日 LeetCode] 225. Implement Stack using Queues

Description:

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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false
​

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).

思路：本题是[每日 LeetCode] 232. Implement Queue using Stacks 镜像题，本题要求用队列结构实现队列的基本操作。由于队列先进先出的特性，考虑只使用一个队列完成操作，具体做法为：对每个栈的 push 操作后，对当前队列逆序，让队头始终保存刚刚压入的元素，如此一来即可实现栈的基本操作。

C++ 代码

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    /** Push element x onto stack. */
    void push(int x) {
        temp.push(x);
        for (int i=0; i<temp.size() - 1; i++){
            temp.push(temp.front());
            temp.pop();
        }
    }    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int tem = temp.front();
        temp.pop();
        return tem;
    }   
    /** Get the top element. */
    int top() {
        return temp.front();
    }
    /** Returns whether the stack is empty. */
    bool empty() {
        return temp.empty();
    }
private:
    queue<int> temp;
};
/**
 * 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();
 * bool param_4 = obj->empty();
 */
​

运行时间：4ms

运行内存：8.9M