[leetcode] 225. Implement Stack using Queues

Posted 2020-08-07

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).

 

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Solution:

使用两个队列qm,qs, qm存储元素，qs作为临时存储:

1)入栈列时直接qm.push

2)出栈时，先将qm中的前N-1个元素push到qs中，然后pop出最后的第N个元素，之后把qs中的元素转移到qm中

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