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
, andis 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)
类似上一题的两个栈实现队列,但是有点不同,具体的见下面的注释:
1 class Stack { 2 public: 3 // Push element x onto stack. 4 void push(int x) { 5 q1.push(x); 6 } 7 8 // Removes the element on top of the stack. 9 void pop() { 10 int tmp = q1.front();//注意这里的实现和双栈实现队列的方式是不相同的 11 q1.pop(); 12 while(!q1.empty()){ 13 q2.push(tmp); 14 tmp = q1.front(); 15 q1.pop(); 16 } 17 swap(q1,q2); 18 } 19 20 // Get the top element. 21 int top() { 22 int tmp = q1.front(); 23 q1.pop(); 24 while(!q1.empty()){ 25 q2.push(tmp); 26 tmp = q1.front(); 27 q1.pop(); 28 } 29 q2.push(tmp); 30 swap(q1, q2);//用swap即可,无须再向上次那样再倒回去 31 return tmp; 32 } 33 34 // Return whether the stack is empty. 35 bool empty() { 36 return q1.empty(); 37 } 38 private: 39 queue<int> q1; 40 queue<int> q2; 41 };