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).

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

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