155 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.
Two stacks are used to achieve this, one stack is used to store the data that push in in order, and the other is used to store the smallest value that has occurred.