225. Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

用队列实现栈的功能

C++（2ms）：

class MyStack {
public:
    queue<int> q ;
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x) ;
        for(int i = 0 ; i < q.size()-1 ; i++){
            q.push(q.front()) ;
            q.pop() ;
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int t = q.front() ;
        q.pop() ;
        return t ;
    }
    
    /** Get the top element. */
    int top() {
        return q.front() ;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty() ;
    }
};

/**
 * 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();
 */