LeetCode Implement Queue using Stacks

Posted 2020-06-30

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)
• 没什么好说的，学过数据结构的应该都能看的懂

• class Queue {
  public:
      stack<int> iSta1, iSta2;
      // Push element x to the back of queue.
      void push(int x) {
          iSta1.push(x);
      }
  
      // Removes the element from in front of queue.
      void pop(void) {
          int i = 0;
          while (!iSta1.empty())
          {
              i = iSta1.top();
              iSta2.push(i);
              iSta1.pop();
          }
          iSta2.pop();
          while (!iSta2.empty())
          {
              i = iSta2.top();
              iSta1.push(i);
              iSta2.pop();
          }
      }
  
      // Get the front element.
      int peek(void) {
          int i = 0,j=0;
          while (!iSta1.empty())
          {
              i = iSta1.top();
              iSta2.push(i);
              iSta1.pop();
          }
          j = i;
          while (!iSta2.empty())
          {
              i = iSta2.top();
              iSta1.push(i);
              iSta2.pop();
          }
          return j;
      }
  
      // Return whether the queue is empty.
      bool empty(void) {
          return iSta1.empty();
      }
  };