LeetCode225—Implement Stack using Queues-蒲公英云

LeetCode225—Implement Stack using Queues

原题链接

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

要让队首的元素到对尾去，因为stack是先进后出，没次入队之后将队首元素再入队。

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