225. Implement Stack using Queues-蒲公英云

225. Implement Stack using Queues

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).

题意：用队列（先进先出）实现栈（先进后出）。

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