[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 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).

 

两个queue实现stack。

q1移入q2,留下一个，就是top和pop的那个数了。

 1 class Stack {
 2 public:
 3     queue<int> q1,q2;
 4     // Push element x onto stack.
 5     void push(int x) {
 6         q1.push(x);
 7     }
 8 
 9     // Removes the element on top of the stack.
10     void pop() {
11         while(q1.size()>1)
12         {
13             int tmp = q1.front();
14             q1.pop();
15             q2.push(tmp);
16         }
17         q1.pop();
18         while(q2.size()>0)
19         {
20             int tmp = q2.front();
21             q2.pop();
22             q1.push(tmp);
23         }
24     }
25 
26     // Get the top element.
27     int top() {
28         while(q1.size()>1)
29         {
30             int tmp = q1.front();
31             q1.pop();
32             q2.push(tmp);
33         }
34         return q1.front();
35     }
36 
37     // Return whether the stack is empty.
38     bool empty() {
39         if(q1.empty() && q2.empty()) return true;
40         else return false;
41     }
42 };

 

转载于:https://www.cnblogs.com/Sean-le/p/4814081.html