Implement Queue using Stacks（用栈实现队列）

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).

• 题目解析：
• 用堆栈stack实现队列，仅仅能使用堆栈的操作，push()，pop（）。empty().

• 方法：
• 用两个堆栈实现。代码例如以下：

• class Queue {
  public:
      stack<int> st1;
      stack<int> st2;
      bool st1_use=1;  
      bool st2_use=0;  
      // Push element x to the back of queue.
      void push(int x) {
          if(st1_use==1)
          st1.push(x);
          else if(st2_use==1)
          st2.push(x);
      }
  
      // Removes the element from in front of queue.
      void pop(void) {
          if(st1.empty()&&st2.empty())  
          exit(0);
          if(st1_use==1&&st2_use==0)
          {
              while(st1.size()>1)
              {
                  int temp=st1.top();
                  st2.push(temp);
                  st1.pop();
              }
              st1.pop();
  			while(st2.size()>0)
              {
                  int temp=st2.top();
                  st1.push(temp);
                  st2.pop();
              }
              st1_use==1;
              st2_use==0;
  			return;
          }
          if(st1_use==0&&st2_use==1)
          {
              while(st2.size()>1)
              {
                  int temp=st2.top();
                  st1.push(temp);
                  st2.pop();
              }
              st2.pop();
  			while(st1.size()>0)
              {
                  int temp=st1.top();
                  st2.push(temp);
                  st1.pop();
              }
              st1_use==0;
              st2_use==1;
  			return;
          }
      }
  
      // Get the front element.
      int peek(void) {
          if(st1_use==1&&st2_use==0)
          {
  			int temp;
              while(st1.size()>0)
              {
                  temp=st1.top();
                  st2.push(temp);
                  st1.pop();
              }
  			
  			while(st2.size()>0)
              {
                  int temp1=st2.top();
                  st1.push(temp1);
                  st2.pop();
              }
              st1_use==1;
              st2_use==0;
  			return temp;
          }
          if(st1_use==0&&st2_use==1)
          {
  			int temp;
              while(st2.size()>0)
              {
                  int temp=st2.top();
                  st1.push(temp);
                  st2.pop();
              }
  			
  			while(st1.size()>0)
              {
                  int temp1=st1.top();
                  st2.push(temp1);
                  st1.pop();
              }
              st1_use==0;
              st2_use==1;
  			return temp;
          }
      }
  
      // Return whether the queue is empty.
      bool empty(void) {
          if(st1_use==1&&st1.empty())
          return true;
          if(st2_use==1&&st2.empty())
          return true;
          return false;
      }
  
  };