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:
    MyStack() {
    }
    
    void push(int x) {
        D.push_back(x);
    }
    
    int pop() {
        int n = D.size();
        for(int i =0;i<n-1;i++){
            int val = *D.begin();
            D.pop_front();
            D.push_back(val);
        }
        int res = *D.begin();
        D.pop_front();
        return res;
    }
    
    int top() {
        int val = 0;
        for(int i =0;i< D.size();i++){
            val = *D.begin();
            D.pop_front();
            D.push_back(val);
        }
        return val;
    }
    
    bool empty() {
        return D.empty();
    }
private:
    deque<int> D;
};
/**
 * 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

2: 注意， D.begin() 返回的是iterator， 需要*