Implementing Stacks 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). 

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

 

Analyse: Using two queues to store the information. q is used to mock the stack while help is used to store temporary values.

1. Time Exceeded Limit Version

　 It's because transfering temporary queue to the original queue costs a lot of time. 

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

2. Set a 2-size array to respectively represent the two queue. When it comes to copy the temporary queue to the original queue, just switch the index.

    Runtime: 0ms.

 1 class Stack {
 2 private:
 3     queue<int> q[2];
 4     int index = 0;
 5 public:
 6     // Push element x onto stack.
 7     void push(int x) {
 8         q[index].push(x);
 9     }
10 
11     // Removes the element on top of the stack.
12     void pop() {
13         while(q[index].size() > 1){ //transfer all other elements into the helper queue
14             q[1 - index].push(q[index].front());
15             q[index].pop();
16         }
17         q[index].pop();
18         index = 1 - index;
19     }
20 
21     // Get the top element.
22     int top() {
23         while(q[index].size() > 1){ //transfer all other elements into the helper queue
24             q[1 - index].push(q[index].front());
25             q[index].pop();
26         }
27         int result = q[index].front();
28         q[1- index].push(result);
29         q[index].pop();
30         index = 1 - index;
31         return result;
32     }
33 
34     // Return whether the stack is empty.
35     bool empty() {
36         return q[index].empty();
37     }
38 };

 

posted @ 2015-08-18 03:11  amazingzoe  阅读(125)  评论(0)  编辑  收藏  举报