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。