225. Implement Stack using Queues

问题：

设计数据结构，使用queue实现stack。

实现以下功能：

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
 

Constraints:

1 <= x <= 9
At most 100 calls will be made to push, pop, top, and empty.
All the calls to pop and top are valid.

解法：单队列->实现队列

参考labuladong解析

重点在：

查栈顶元素top 和出栈pop

使用额外变量，记录top元素：top_ele

循环queue内所有元素，剩下最后一个即为要求栈顶元素top

同时更新 top_ele 为倒数第二个元素。

参考代码：

 1 class MyStack {
 2 public:
 3     /** Initialize your data structure here. */
 4     MyStack() {
 5         
 6     }
 7     
 8     /** Push element x onto stack. */
 9     void push(int x) {
10         q.push(x);
11         top_ele = x;
12     }
13     
14     /** Removes the element on top of the stack and returns that element. */
15     int pop() {
16         int size = q.size();
17         while(size>2) {
18             q.push(q.front());
19             q.pop();
20             size--;
21         }
22         top_ele = q.front();
23         q.pop();
24         q.push(top_ele);
25         int res = q.front();
26         q.pop();
27         return res;
28     }
29     
30     /** Get the top element. */
31     int top() {
32         return top_ele;
33     }
34     
35     /** Returns whether the stack is empty. */
36     bool empty() {
37         return q.empty();
38     }
39 private:
40     queue<int> q;
41     int top_ele;
42 };
43 
44 /**
45  * Your MyStack object will be instantiated and called as such:
46  * MyStack* obj = new MyStack();
47  * obj->push(x);
48  * int param_2 = obj->pop();
49  * int param_3 = obj->top();
50  * bool param_4 = obj->empty();
51  */