[leetcode 234]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)。
• 使用两个栈，模拟队列操作
• AC代码

• class Queue {
  public:
      // Push element x to the back of queue.
      void push(int x) {
          s1.push(x);
      }
  
      // Removes the element from in front of queue.
      void pop(void) {
          if(s2.empty())
          {
              while(!s1.empty())
              {
                  s2.push(s1.top());
                  s1.pop();
              }
          }
          s2.pop();
      }
  
      // Get the front element.
      int peek(void) {
          if(s2.empty())
          {
              while(!s1.empty())
              {
                  s2.push(s1.top());
                  s1.pop();
              }
          }
          return s2.top();
      }
  
      // Return whether the queue is empty.
      bool empty(void) {
         return s1.empty()&&s2.empty();
      }
  private :
      stack<int> s1;
      stack<int> s2;
  };

  
  

  其他Leetcode题目AC代码：https://github.com/PoughER/leetcode