225. Implement Stack using Queues -E

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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

A:

就是简单的，每次都pop()出来，再放到最后去了

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        dq.push_back(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        deque<int> dq2;
        while(dq.size()>1)
        {
            int v = dq.front();
            dq.pop_front();
            dq2.push_back(v);
        }
        int res = dq.front();
        dq = dq2;
        return res;
    }
    
    /** Get the top element. */
    int top() {
        deque<int> dq2;
        int res;
        while(dq.size()>0)
        {
            res = dq.front();
            dq.pop_front();
            dq2.push_back(res);
        }
        dq = dq2;
        return res;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return dq.empty();
    }
    deque<int> dq;
};

/**
 * 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();
 */

Mistakes:

1: 第一遍的时候，理解成是用2个Stack来表示Queue了    太TMD搞笑（Diu Ren)了