leetcode——232——Implement Queue using Stacks

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.

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 Queue {
  public:
      stack<int> stack1;
      stack<int> stack2;
      // Push element x to the back of queue.
      void push(int x) {
          
          stack1.push(x);
      }
  
      // Removes the element from in front of queue.
      void pop(void) {
          if(stack2.size()<=0)
          {
              while(stack1.size()>0)
              {
                  int data = stack1.top();
                  stack1.pop();
                  stack2.push(data);
              }
             
          }
           stack2.pop();
      }
  
      // Get the front element.
      int peek(void) {
           if(stack2.size()<=0)
          {
              while(stack1.size()>0)
              {
                  int data = stack1.top();
                  stack1.pop();
                  stack2.push(data);
              }
            
          }
           return stack2.top();
      }
  
      // Return whether the queue is empty.
      bool empty(void) {
          return (stack1.size()<=0) && (stack2.size()<=0);
      }
  };

  
  

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