225. 用队列实现栈 【可用两个队列或一个队列实现。把队列当环。】【队列： 前端进， 后端出。 栈： 后端进，后端出】

225. 用队列实现栈

用两个队列

class MyStack:
         # 栈：  后端入 后端出
        # 队列：前端出, 后端出

    def __init__(self):
        # q1: 存储栈内元素
        # q2: 入栈操作的辅助队列
        self.q1 = collections.deque()
        self.q2 = collections.deque()

    def push(self, x: int) -> None:
        # 将元素入队到 q2
        # 将 q1 全部元素取出 并入队到 q2
        self.q2.append(x)
        while self.q1:
            self.q2.append(self.q1.popleft())

        self.q1, self.q2 = self.q2, self.q1  # 交换       


    def pop(self) -> int: # q1 前端
        return self.q1.popleft()

    def top(self) -> int:
        return self.q1[0]

    def empty(self) -> bool:
        return not self.q1


# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()

class MyStack {
public:
    queue<int> queue1;
    queue<int> queue2;
    MyStack() {

    }
    
    void push(int x) {
        queue2.push(x);
        while (!queue1.empty()){
            queue2.push(queue1.front());
            queue1.pop();
        }
        swap(queue1, queue2);
    }
    
    int pop() {
        int r = queue1.front(); queue1.pop();
        return r;
    }
    
    int top() {
        return queue1.front();
    }
    
    bool empty() {
        return queue1.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();
 */

用一个队列

class MyStack:

    def __init__(self):
        self.q = collections.deque()

    def push(self, x: int) -> None: # 获取 之前的元素个数 调位置
        n = len(self.q)
        self.q.append(x)
        for _ in range(n):
            self.q.append(self.q.popleft())

    def pop(self) -> int:
        return self.q.popleft()

    def top(self) -> int:
        return self.q[0]

    def empty(self) -> bool:
        return not self.q



# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()

class MyStack {
public:
    queue<int> q;
    MyStack() {

    }
    
    void push(int x) {
        int n = q.size();
        q.push(x);
        for (int i = 0; i < n; ++i){
            // int r = q.front(); q.pop();
            // q.push(r);
            q.push(q.front());
            q.pop();
        }      

    }
    
    int pop() {
        int r = q.front(); q.pop();
        return r;
    }
    
    int top() {
        return q.front();
    }
    
    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();
 */