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).

解题思路：

    利用两个栈来模拟队列的操作，一个栈负责push，一个栈负责pop，定义一个操作（如：peek()）负责数据的转移。堆栈的顶是其中一个栈的top。

class MyQueue {
public:
     stack input, output;
    /** 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() {      //pop出的数据永远在栈output 的top端
        peek();
        int ret = output.top();
        output.pop();
        return ret;
    }
    
    /** Get the front element. */
    int peek() {
        if(output.empty())  //如果栈output的数据全部被pop，就可以再从input转移数据
        {
            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();
    }
};

/**
 * 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();
 */