225. Implement Stack using Queues

# Easy

Three methods:

1. straight froward method: use queue 1 as main storage and queue 2 as helper. Use q2 when do pop().

2. smart method: use two queues, q1 stores all elements except top element, q2 only stores top element.

3. one queue method: only complicated when doing push(), change the order to the stack order.

reference: https://www.cnblogs.com/grandyang/p/4568796.html

C++方法一

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q1.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        while(q1.size() != 1) {
            int tmp = q1.front();
            q1.pop();
            q2.push(tmp);
        }
        int res = q1.front();
        q1.pop();
        while(q2.size() != 0) {
            int tmp = q2.front();
            q2.pop();
            q1.push(tmp);
        }
        return res;
    }
    
    /** Get the top element. */
    int top() {
        int res = pop();
        q1.push(res);
        return res;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty();
    }
    
private:
    queue<int> q1, q2;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */