[leetcode] 232. Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• • You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
  • Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
  • You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

 

Solution:

 1 class Queue {
 2 public:
 3     stack<int> stkin;
 4     stack<int> stkout;
 5     // Push element x to the back of queue.
 6     void push(int x) 
 7     {
 8         stkin.push(x);    
 9     }
10 
11     // Removes the element from in front of queue.
12     void pop(void) 
13     {
14         if (stkout.empty())
15         {
16             while (!stkin.empty())
17             {
18                 int temp = stkin.top();
19                 stkin.pop();
20                 stkout.push(temp);
21             }
22         }
23         if (!stkout.empty())
24             stkout.pop();
25     }
26 
27     // Get the front element.
28     int peek(void) 
29     {
30         if (stkout.empty())
31         {
32             while (!stkin.empty())
33             {
34                 int temp = stkin.top();
35                 stkin.pop();
36                 stkout.push(temp);
37             }
38         }
39         
40         return stkout.top();
41     }
42 
43     // Return whether the queue is empty.
44     bool empty(void) 
45     {
46         return stkin.empty() && stkout.empty();    
47     }
48 };

 

转载于:https://www.cnblogs.com/ym65536/p/5496272.html