Min Stack

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.

This problem is included in CC150 (See 3.2 in Chapter 3). Further improvement could be save the counter of duplicates (the current version push some duplicates into two stacks).

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