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).

• class Stack {
  public:
      queue<int> queue1;
      queue<int> queue2;
      // Push element x onto stack.
      void push(int x) {
          if(queue1.empty())
          {
              queue1.push(x);
              while(!queue2.empty())
              {
                  int data = queue2.front();
                  queue2.pop();
                  queue1.push(data);
              }
              
          }
          else{
              queue2.push(x);
              while(!queue1.empty())
              {
                  int data = queue1.front();
                  queue1.pop();
                  queue2.push(data);
              }
          }
      }
  
      // Removes the element on top of the stack.
      void pop() {
          if(!queue1.empty())
              queue1.pop();
          if(!queue2.empty())
              queue2.pop();
          
      }
  
      // Get the top element.
      int top() {
          if(!queue1.empty())
             return queue1.front();
           if(!queue2.empty())
             return queue2.front();
          
      }
  
      // Return whether the stack is empty.
      bool empty() {
          return queue1.empty()&&queue2.empty();
      }
  };