232. Implement Queue using Stacks-蒲公英云

232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

push(x) — Push element x to the back of queue.
pop() — Removes the element from in front of queue.
peek() — Get the front element.
empty() — Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

You must use only standard operations of a stack — which means only push to top, peek/pop from top, size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

题意：用栈（先进后出）实现队列（先进先出）。

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
    }
    /** Push element x to the back of queue. */
    void push(int x) {
        _input.push(x);
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(_output.empty())
        {
            while(!_input.empty())
            {
                _output.push(_input.top());
                _input.pop();
            }            
        }
        int temp = _output.top();
        _output.pop();
        return temp;
    }
    /** Get the front element. */
    int peek() {
        if(_output.empty())
        {
            while(!_input.empty())
            {
                _output.push(_input.top());
                _input.pop();
            }            
        }
        return _output.top();
    }
    /** Returns whether the queue is empty. */
    bool empty() {
        return _input.empty() && _output.empty();
    }
private:    
    stack<int> _input;
    stack<int> _output;
};
/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */