155 Min Stack

来源：Leetcode

原帖：https://oj.leetcode.com/problems/min-stack/

题目：
Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.

代码：

 class MinStack {  
 public:  
   void push(int x) {  
     s.push(x);  
     if (min.empty()) {  
       min.push(x);  
     } else if (x <= min.top()) {  
       min.push(x);  
     }  
   }  
   
   void pop() {  
     if (s.empty()) {  
       cerr << "stack is empty" << endl;  
     }  
     if (s.top() == min.top()) {  
       min.pop();  
     }  
     s.pop();  
   }  
   
   int top() {  
     return s.top();  
   }  
   
   int getMin() {  
     assert(!min.empty());  
     return min.top();  
   }  
 private:  
   stack<int> s;  
   stack<int> min;  
 };