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 };