[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 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
, andis 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 };