232. Implement Queue using Stacks -E

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).

Accepted

A:
  -----use two stack to simulate ----------

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        sin.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if(sout.empty())
        {
            while(not sin.empty())
            {
                int v = sin.top();
                sin.pop();
                sout.push(v);
            }
        }
        int v = sout.top();
        sout.pop();
        return v;
    }
    
    /** Get the front element. */
    int peek() {
        if(sout.empty())
        {
            while(not sin.empty())
            {
                int v = sin.top();
                sin.pop();
                sout.push(v);
            }
        }
        return sout.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return sin.empty() && sout.empty();
    }
private:
    stack<int> sin;
    stack<int> sout;
};

/**
 * 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();
 */