232. Implement Queue using Stacks

Posted 2022-08-31 wentiliangkaihua

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

  class MyQueue 
  
      /** Initialize your data structure here. */
      public MyQueue() 
          
      
      
      /** Push element x to the back of queue. */
      public void push(int x) 
          int i = s1.size();
          if(i == 0)
              s1.push(x);
          
          else
              while(i > 0)
              s2.push(s1.pop());
              i--;
              
           s2.push(x);
           while(!s2.empty())
              s1.push(s2.pop());
                          
          
  
      
      
      /** Removes the element from in front of queue and returns that element. */
      public int pop() 
          return s1.pop();
      
      
      /** Get the front element. */
      public int peek() 
          return s1.peek();
      
      
      /** Returns whether the queue is empty. */
      public boolean empty() 
          return s1.empty();
      
      private Stack<Integer> s1 = new Stack<Integer>();
      private Stack<Integer> s2 = new Stack<Integer>();
  
  
  /**
   * Your MyQueue object will be instantiated and called as such:
   * MyQueue obj = new MyQueue();
   * obj.push(x);
   * int param_2 = obj.pop();
   * int param_3 = obj.peek();
   * boolean param_4 = obj.empty();
   */

  也没什么难度，只要掌握stack是FILO，queue是FIFO即可，用两个stack作为辅助进行操作的实现。