LeetCode 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).

class Stack {

private:

    queue<int> que[2];

    int current;

    

    void transfer(queue<int>& que1, queue<int>& que2, int n) {

        for (int i=0; i<n; i++) {

            que2.push(que1.front());

            que1.pop();

        }

    } 

public:

    Stack() : current(0){}

    // Push element x onto stack.

    void push(int x) {

        que[current].push(x);

    }



    // Removes the element on top of the stack.

    void pop() {

        int nums = que[current].size();

        if (nums == 0) {

            return;

        }

        transfer(que[current], que[1 - current], nums - 1);

        que[current].pop();

        current = 1 - current;

    }



    // Get the top element.

    int top() {

        if (empty()) {

            return 0;

        }

        return que[current].back();

    }



    // Return whether the stack is empty.

    bool empty() {

        return que[current].empty();

    }

};

 也可以使用一个队列来完成，不过对于队列来说一个和两个没什么区别只是个头指针而已。