[LintCode] 带最小值操作的栈

 class MinStack {
 public:
     MinStack() {
         // do initialization if necessary
     }

     void push(int number) {
         // write your code here
         if (minimum.empty() || number <= minimum.top())
             minimum.push(number);
         data.push(number);
     }

     int pop() {
         // write your code here
         if (minimum.top() == data.top())
             minimum.pop();
         int elem = data.top();
         data.pop();
         return elem;
     }

     int min() {
         // write your code here
         return minimum.top();
     }
 private:
     stack<int> data;
     stack<int> minimum;
 };