【LeetCode】225 - 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).

 1 class Stack {
 2 public:
 3     // Push element x onto stack.
 4     void push(int x) {
 5         q.push(x);
 6     }
 7
 8     // Removes the element on top of the stack.
 9     void pop() {
10         queue<int> temp;
11         while(q.size()!=1){
12             int x=q.front();
13             q.pop();
14             temp.push(x);
15         }
16         q.pop();
17         while(!temp.empty()){
18             int x=temp.front();
19             temp.pop();
20             q.push(x);
21         }
22     }
23
24     // Get the top element.
25     int top() {
26         queue<int> temp;
27         while(q.size()!=1){
28             int x=q.front();
29             q.pop();
30             temp.push(x);
31         }
32         int x=q.front();
33         q.pop();
34         temp.push(x);
35         while(!temp.empty()){
36             int x=temp.front();
37             temp.pop();
38             q.push(x);
39         }
40         return x;
41     }
42
43     // Return whether the stack is empty.
44     bool empty() {
45         return q.empty();
46     }
47 private:
48     queue<int> q;
49 };