LeetCode 225 Implement Stack using Queues 用队列实现栈

1、两个队列实现，始终保持一个队列为空即可

 class MyStack {
 public:
     /** Initialize your data structure here. */
     MyStack() {

     }

     /** Push element x onto stack. */
     void push(int x) {
         if(que1.empty())
         {
             que1.push(x);
             while(!que2.empty())
             {
                 int val=que2.front();
                 que2.pop();
                 que1.push(val);
             }
         }
         else
         {
             que2.push(x);
             while(!que1.empty())
             {
                 int val=que1.front();
                 que1.pop();
                 que2.push(val);
             }
         }
     }

     /** Removes the element on top of the stack and returns that element. */
     int pop() {
         if(que1.empty())
         {
             int val=que2.front();
             que2.pop();
             return val;
         }
         if(que2.empty())
         {
             int val=que1.front();
             que1.pop();
             return val;
         }
     }

     /** Get the top element. */
     int top() {
         if(que1.empty())
             return que2.front();
         if(que2.empty())
             return que1.front();
     }

     /** Returns whether the stack is empty. */
     bool empty() {
         return que1.empty()&&que2.empty();
     }
 private:
     queue<int> que1;
     queue<int> que2;
 };

 /**
  * Your MyStack object will be instantiated and called as such:
  * MyStack obj = new MyStack();
  * obj.push(x);
  * int param_2 = obj.pop();
  * int param_3 = obj.top();
  * bool param_4 = obj.empty();
  */

2、一个队列实现栈

 class MyStack {
 public:
     /** Initialize your data structure here. */
     MyStack() {

     }

     /** Push element x onto stack. */
     void push(int x) {
         que.push(x);
         for(int i=;i<que.size()-;++i)
         {
             que.push(que.front());
             que.pop();
         }
     }

     /** Removes the element on top of the stack and returns that element. */
     int pop() {
         int val=que.front();
         que.pop();
         return val;
     }

     /** Get the top element. */
     int top() {
         return que.front();
     }

     /** Returns whether the stack is empty. */
     bool empty() {
         return que.empty();
     }
 private:
     queue<int> que;
 };

 /**
  * Your MyStack object will be instantiated and called as such:
  * MyStack obj = new MyStack();
  * obj.push(x);
  * int param_2 = obj.pop();
  * int param_3 = obj.top();
  * bool param_4 = obj.empty();
  */