[LintCode] 用栈实现队列

 class Queue {
 public:
     stack<int> stack1;
     stack<int> stack2;

     Queue() {
         // do intialization if necessary
     }

     void push(int element) {
         // write your code here
         stack1.push(element);
     }

     int pop() {
         // write your code here
         if (stack2.empty()) {
             while (!stack1.empty()) {
                 int elem = stack1.top();
                 stack1.pop();
                 stack2.push(elem);
             }
         }
         int elem = stack2.top();
         stack2.pop();
         return elem;
     }

     int top() {
         // write your code here
         if (stack2.empty()) {
             while (!stack1.empty()) {
                 int elem = stack1.top();
                 stack1.pop();
                 stack2.push(elem);
             }
         }
         return stack2.top();
     }
 };