Implement_Queue_using_Stacks.cpp

// Source : https://leetcode-cn.com/problems/implement-queue-using-stacks/description/
// Number : 232
// Author : HL
// Date   : 2018-09-11
// Kill   : 100.00%

/********************************************************************************** 
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).
**********************************************************************************/

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int temp;
        if(s2.empty())
        {
            while(!s1.empty())
            {
                temp = s1.top();
                s2.push(temp);
                s1.pop();
            }
        }
        temp = s2.top();
        s2.pop();
        return temp;
    }
    
    /** Get the front element. */
    int peek() {
        int temp;
        if(s2.empty())
        {
            while(!s1.empty())
            {
                temp = s1.top();
                s2.push(temp);
                s1.pop();
            }
        }
        temp = s2.top();
        return temp;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
private:
    stack<int> s1,s2;
};

/**
 * 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();
 */