Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
<li>push(x) -- Push element x onto stack.</li>
<li>pop() -- Removes the element on top of the stack.</li>
<li>top() -- Get the top element.</li>
<li>getMin() -- Retrieve the minimum element in the stack.</li>
Example:
MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); --> Returns -3. minStack.pop(); minStack.top(); --> Returns 0. minStack.getMin(); --> Returns -2.