[LeetCode] Implement Stack using Queues

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 may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
• 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 -- which means only push to back, pop from front, size, and is empty operations are valid.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

水题。用两个队列来模拟。

 1 class Stack {

 2 private:

 3     queue<int> que[2];

 4     int cur = 0;

 5     

 6 public:

 7     // Push element x onto stack.

 8     void push(int x) {

 9         que[cur].push(x);

10     }

11 

12     // Removes the element on top of the stack.

13     void pop(void) {

14         while (que[cur].size() > 1) {

15             que[1 - cur].push(que[cur].front());

16             que[cur].pop();

17         }

18         que[cur].pop();

19         cur = 1 - cur;

20     }

21 

22     // Get the top element.

23     int top(void) {

24         while (que[cur].size() > 1) {

25             que[1 - cur].push(que[cur].front());

26             que[cur].pop();

27         }

28         int top = que[cur].front();

29         que[1 - cur].push(que[cur].front());

30         que[cur].pop();

31         cur = 1 - cur;

32         return top;

33     }

34 

35     // Return whether the stack is empty.

36     bool empty(void) {

37         return que[cur].empty();

38     }

39 };