Min Stack

Description：

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.

Code：

 1 class MinStack {
 2 public:
 3     void push(int x) {
 4         m_stack.push(x);
 5         if (min_num.empty() || x<min_num.top())
 6            min_num.push(x);
 7         else
 8            min_num.push(min_num.top());
 9     }
10 
11     void pop() {
12         assert( !m_stack.empty() && !min_num.empty() );
13         m_stack.pop();
14         min_num.pop();
15     }
16 
17     int top() {
18             assert( !m_stack.empty() );
19             return m_stack.top();
20     }
21 
22     int getMin() {
23         return min_num.top();
24     }
25 private:
26     stack<int>m_stack;
27     stack<int>min_num;
28 };